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BMW Airhead Motorcycles.
Carburetors: how to properly synchronize them
The Whole Story....I HOPE!

Copyright, 2014, R. Fleischer

Read this article completely through, slowly & carefully,
at least twice, before you attempt ANYthing.  This article
is purposely long, to inform you of every possible detail.

Preliminary information section:

It was in November of 1971 that BMW first...& almost lastly....recommended OPENING the throttle SLIGHTLY during very cold starts. We all know that this can be a necessity depending on the bike, temperature (does NOT have to be quite cold), & how the carburetors are adjusted.  BMW has dropped the recommendation of slightly opening throttle.  SHAME.


Do not lubricate the throttle cables.   Originally, only cables from 1978 & later were thought to have an inner lining that self-lubricates, wearing very slowly over large mileage. However, it appears there is some controversy over this & lined cables were phased-in immediately after the /5 models.  Earlier cables (such as the original /5 types) could be lubricated, but have likely already been replaced.  All throttle & clutch cables that have been replaced, are extremely likely to be the newer DO NOT LUBRICATE lined-types.  Throttle cables should normally be replaced at reasonable intervals, perhaps 80,000 miles, on-condition.   If you lubricate the lined-type BMW cables, the liner may swell, making things much worse.    Lubrication of the cable innards is generally a last-ditch effort, when you do not have new cables at hand.  It is UNCLEAR if the inner lining will be OK & NOT swell if you lubricated it with silicone oil.  NEVER use WD40!

Replace cables if fraying (typically at the left carb!); or, if the throttle feels draggy or catchy/grumbly (if it isn't because of badly worn gears at the throttle assembly at the handlebars).  Some recently shipped BMW cables are lousily-made at the ends...although this is mostly clutch cables.  If you have such cables, fix the ends!  They MUST operate smoothly at the barrel ends and into what the barrel fits into!   Failure to ensure a smoothly rotating "cable end barrel" & to have a good bushing in the Clutch lever at the handlebars, & a smoothly rotating "cable end barrel" at the rear of the transmission.....has caused many problems with cables.  In some instances, a tiny bit of filing of the barrel, or the fork area at the transmission clutch lever, has been necessary.   Lubrication of the barrels should be by MOS2 (Moly) grease or moly oil.  ONE big reason for cable problems is failure to lubricate the barrels at the carbs; I do it after EVERY wash job, after drying. If these barrels are not able to easily rotate, they put a fair amount of sharp-bending-side-force on the steel strands...weakening them, and soon they snap, one by one.

If someone makes a habit of bending the left throttle cable at the carburetor while removing & replacing the oil level dipstick, the throttle cable may fail, sometimes rather quickly. After just ONE strand breaks, I have seen the rest go in as little as 200 miles.  A bend at the outer jacket at the carburetor can cause friction.  Keep in mind that worn cables, friction, etc., means more throttle effort for your right hand, & it means MORE WEAR ON THE THROTTLE GEARS!

>>>Do NOT install carburetor cables with lots of ties, nor sharp curves.  ONE tie for the clutch cable.  The cables must be free to move about, particularly as you steer the motorcycle, & VERY particularly the clutch cable.

WHY synchronize carburetors?
What's the big deal?  Why so many opinions?  Why are some books like Clymers & Haynes so misleading (or just plain WRONG).  Why Snowbum's advice & NOT them? (I can't give you any outstanding reasons on the last part, except maybe to listen to your elders who have lots of experience).

Your BMW boxer engine has two pistons going in & out of the cylinders at the SAME time, same direction, same amount.   While the engine would SEEM to be perfectly balanced mechanically, that is NOT so.   The crankshaft has weights on it, & they are not perfect at  balancing the crankshaft at all rpm, nor at all parts of any piston stroke.   Parts may not be perfectly balanced with regards to weight & reciprocating offset mass.  That WILL cause some vibration, usually showing up in a narrow rpm band.  Imbalance in the clutch
assembly is another cause for vibration.  MANY a clutch assembly is also not assembled correctly per marks, or marks are lost. The cylinders are NOT opposite each other, one is BEHIND the other, producing what is called 'a rocking couple', which is a twisting cyclical vibration.  Actual internal friction & combustion pressures between cylinders may vary, & there are MANY other variables, such as small ignition timing differences, differences in valve settings & valve seating areas, camshaft lobes; engine wear....and more.

As the timing chain stretches (actually the wear on the crankshaft sprocket & guides/tensioner are the most common wear items, not the chain), this affects cam timing & ignition.  While the stock ignition is easily adjusted, it does NOT do so for both cylinders in a perfect way.  There is jerky timing chain operation, particularly as the noted parts wear, causing IRREGULAR timing between cylinders, & in the same cylinder too.  This is quite easy to see when you check the ignition timing & advance at the 'flywheel', with a stroboscopic light.  In addition, as the the chain & its tensioner/guides wear, the CAMSHAFT will LAG, with respect to the crankshaft. 

External to the engine core are variations in the carburetors & the cables operating those carburetors. MOST of these variations can be adjusted to be perfect...or, close to that.   

Because of all these & other factors, the BMW boxer Airhead engine can be rather sensitive to carburetor adjustments.  This is particularly so for the 1980's & later models that were factory tuned to run leaner & also have lighter flywheel-clutch assemblies (the flywheel from 1981 is called a Clutch Carrier, but it IS a flywheel).

An imbalance in synchronization of the cables & carburetors, together with effects &  differences described above; and, even such as diaphragm differences in the carburetors (I could make a long list of things having effects)....will cause engine roughness or vibration, sometimes in a narrow band & sometimes at two different narrow bands of rpm.    Imbalance in the cables & idle mixtures/idle rpm between cylinders will typically result in slight rocking or stumbling as one comes off the idle stops, particularly if doing so gently.   Occasionally things are worse if one of the imbalance points for the various items discussed happens to be at the natural balance frequency of the crankshaft assembly.  That TENDS to be AROUND 3900-4400 rpm, with many centered around 4250 rpm.   

If the pressure in a combustion chamber/cylinder is different in the left cylinder, compared to the right cylinder; the engine will rock back & forth, sometimes a very little bit, sometimes more.   As rpm is raised, the rocking becomes a faster pulsing.  Further increase in rpm & it becomes a vibration or tingling in the bars.  It is entirely possible & COMMON for combustion pressures to be closely the same at some particular rpm & throttle setting & NOT be the same at any other throttle setting INCLUDING idle.

Properly synchronizing the carburetors, assuming all else is even close to reasonably OK, will give smoother throttle operation, particularly noticeable at the just-off-idle area, & will definitely also reduce higher rpm vibration.  Just-off-idle is important, because on gentle take-offs, THAT is the point that even slight engine stumbling would be exceptionally annoying.   If the carburetors are NOT in good condition & properly adjusted & synchronized, the engine may well have very poor throttle feel, the engine might stumble, it might even backfire, it might use an excessive amount of fuel, & it might well cause a lot of  'funny vibration'. Commonly seen is unstable idle and in particular a large increase in idle rpm after full warm-up.  Sometimes seen is an engine that is difficult to start, or perhaps act very lean, etc.    If the idle mixture is improperly set, it is not unusual to see red exhaust pipes near the heads.


If you do not have the rest of your engine properly adjusted, such as ignition timing & valve clearances; if you have problems with ignition wiring or coils or points or condenser or spark plugs or spark plug caps; if the floats & diaphragms, passageways, jets, & jet needle (in particular)
are not in good condition..... then you are totally wasting your time playing with carburetor synchronization. 
The carburetors are the LAST...LAST!!...item to adjust!   You must NOT adjust your carburetors until the ignition &  valves are known to be set properly, the floats are known good, float adjustment is proper...including that the fuel level in the bowls is known to be correct.   Diaphragms must be known good.  In addition, the butterflies in the CV carbs & their enrichener parts ABSOLUTELY MUST be properly assembled.    The CV carb butterflies can be installed backwards, & that will cause you endless problems with idle adjustments.  Some enrichener               parts are/were WRONGLY marked by Bing....YES....that little punch prick on the shaft has been known to be mis-marked.  Do NOT depend on prior shop or owner work!


You should have a relatively clean air cleaner (you do not need a totally clean air cleaner) & no vacuum leaks at the carburetor-to-head adapters (TEST for that, it is SIMPLE and EASY to do).   If your cables are worn enough, or kinked, or routed wrongly, your carburetors will NOT STAY IN SYNCHRONIZATION!!    If the throttle at the bars has badly worn teeth at its barrel & cam, those should be replaced, as they WILL cause stiff irregular throttle action.  Once the throttle gear & associated cam gear (that has the chain thing-y) are worn enough, the teeth WILL won't like that.  Those parts also need to be lubricated & operating smoothly.   BTW...those parts on earlier models have been updated, & you should consult my other articles for the details; or, a good parts person at a BMW dealership or Independent Servicer. 

I see a fair amount of Airheads that do not have sufficient free-play in the throttle cables (sometimes choke cables and enrichener cables).   If the needed throttle free-play needed is not present, moving the bars may change the idle rpm; the engine is likely to drift higher in rpm, usually as it warms up....(however, that is often a symptom of other problems, including idle mixture adjustments and/or a sticky automatic advance).  Rider's tend to not think about throttle cables.   It is important that SOME free play be in BOTH throttle cable outer jackets as seen at the carburetors, with the throttle at the bars fully turned off, but your hand NOT on it...that is, NOT rotating the throttle further off than its spring-loaded off point.  The throttle return springs are located AT the carburetors.   Conversely, excessive free play in the cables, depending on rider technique, can result in jerky operation at low rpm. If there is enough excessive free-play, the outer throttle outer jacket can ride-up onto the carburetor threaded adapter, causing all sorts of problems.   1/8" is plenty good enough, assuming proper cable routing.

It is beyond the scope of this article...which is already tell you how to check, fix, adjust, & generally be sure everything is correct before you attempt carburetor synchronization.  This article is to tell you HOW to do the synchronization, how to use the simple tools, & how to do the job quickly & accurately.  ANYone can do these things!  You do NOT need to have some sort of vast experience!

What tools do you need? ....and some early information on methods:

You will need a flat blade screwdriver to adjust the idle mixture screws & idle stop (RPM) screws, & perhaps remove a screw plug from a vacuum port (may not be a screw, could be a push-on hose & you might not even have vacuum ports on very early Airheads).
IF using the Shorting Method (Method #2), then this screwdriver should NOT be too short in length & it MUST HAVE a plastic electrically insulated handle.  The metal portion of the screwdriver MUST NOT extend to the end of the hand-area, where you could possibly come in contact with it.

You will need a wrench to fit the cable length adjustment locking nuts.  Some folks have wrenches they like better than the BMW one (I am in that category).  You need a short wrench that fits to avoid rounding the edges of the carburetor cable adjustor lock nuts, which are not overly hardened.    My favorite wrench is a common flat open end type, bent upwards some.

YOU NEED TO DO AT LEAST ONE OF THE TWO methods (#1, or #2) JUST BELOW!...or, you certainly CAN use a combination of both methods!  Some folks do Method #1, quickly, then take a ride, & then finish up with Method #2.   You can certainly do the entire synchronizing with either, or a combination.  Method #2 gives the BEST synchronization. 

Note that if the carburetors were taken apart, for, say, an overhaul, & all the adjustments are now WAY off (you can start with the factory recommended settings though), then any method takes longer.  Once the carburetors ARE synchronized, future adjustments
can be done quickly.

Method #1:  This method uses some sort of vacuum sensing gauge or gauge set.  This can be a water or oil or mercury manometer (Sticks or Stix, etc.); a Walus differential gauge; or an electronic vacuum differential instrument (TwinMax, etc.).  In my opinion, the best carburetor vacuum type synchronizer is the Harmonizer, by Grok; although VERY sensitive.  I believe it is better & much more useful than the TwinMax. 

Here is a snip from my References article:

Carburetor Synchronizer:   Harmonizer.  POSSIBLY THE BEST synchronizer is VERY sensitive...and you might even want something less touchy. The maker of the Grok Harmonizer died in April 2016.  A few units were still available that were made before his death, via his daughter:

If you can't find the specs, details, etc., anyplace, try here:
Latest word, September 2016; a capture from an Advrider thread:

""Thank you all for your messages, we were pretty shocked how quickly those 9 went. I have some good news and some bad news. Bad news: We only have the supplies to sell about 30 more harmonizers of this model. Afterwards it will need to be redesigned due to a sensor being discontinued. This may take upwards of a year or more. My dad was aware of this and had already started researching replacement designs but had not finished. Luckily, he had an engineer friend who has come to our attention and knows a lot of my dad's design secrets! This has all been very confusing/exciting for us to figure out.. mostly confusing, but we are slowly sifting through the information. Good News: We are getting about 20 harmonizers ready to sell here on this forum by Friday. I will put a paypal button in a new reply on this thread. Some time afterwards, we will be putting the remaining 10 up for auction on eBay.
Disclosure:  Snowbum uses a Walus Engineering dual-differential gauge, that has not been available for many years.


If your carburetors do NOT have vacuum ports, these ports can usually be added.   SOME synch tools don't work well on the BMW carburetors, or are too slow in use, etc.  The Unisyn comes to mind here.  It IS usable, if you are very careful; BUT, I don't like using them on Airheads.  I am not a fan of mercury tube or colored alcohol tubes type of synchronizers.

Early CV carburetors that DID have vacuum ports (NOT all did) had the ports sticking out from the carburetor horizontally. All later models had ports, & they stick downwards from the underside.   These later ones often have soft black rubber hoses that lead to the Pulse Air system in the air filter box.   The carburetor vacuum ports have INternal screw threads, easily seen after the hose is pulled off, and the thread is a UNcommon type.  You do NOT have to install a screw (nor washer), unless you are removing the vacuum hose permanently for some reason.  

A popular modification is to remove the rubber hose sections & block their air cleaner area, when removing the Pulse-Air system.  I have mixed feelings on that, & there is NO harm done by leaving the vacuum hoses connected to each other, using the air cleaner area small T-fitting already there, & PLUGGING the rear facing part of that T.   That means NO vacuum port screws to lose.   Disregard folks that tell you that this makes the carburetor action smoother, or that it messes up carburetion.  It does neither.   There is an extensive article on this website that fully describes the Pulse-Air system, how to remove or plug it, & further has information on the tank fumes system & the fuel cutoff system used on the last Airheads:

Method #2:  This method uses spark plug shorting tools.  If you do it correctly, this Method is more accurate than any other.  TWO types of tools are used.  You will need tools I will call shorting adapters (TWO unless dual-plugged; then FOUR; or a switching/shorting 'box' helpful mostly for dual-plugged bikes). One or two  plastic handled (for electrical insulation) screwdriver(s), of any reasonable type; can be flat blade, phillips, etc.   Don't use 3 inch screwdriver(s), they are too short for safety. Do not use wood handled types, there is a chance you could get an electrical shock. DO NOT use a screwdriver whose metal shank comes completely to the hand-end.   While you can imagine quite a few types of spark plug shorting tools, AND there is a photo of some types farther down in this article you are reading to give you some ideas.....a version of these tools are available here: It is basically the same as a tool I have been using for many years.  It is a piece of spoke from a wheel, & a spark plug screw-on barrel fitting.  I have also used threaded spoke fittings.  Wheel spokes have the correct thread, so a spoke fitting can be used, or, the spark plug screw-barrel, same thread.   You may decide on other methods for dual-plugged bikes. 

First, I will describe what these tools are for, then you can decide on what to make...or purchase. 

What you are going to do, in part of the synchronization work, if you select my Method #2 (as opposed to vacuum methods); is to first run the engine at idle... & afterwards also just off-idle (above idle a small amount), ...making adjustments to the carburetors.  You will listen to the sound of the engine while you run the engine with both spark plugs operating.   Then again with you shorting out first one spark plug (assuming standard two plugs engine), then the other, doing only one plug shorting at a time, & listening to the engine during these shorting's. You are, in effect, shutting down one cylinder, then allowing it to operate while you shut down the other cylinder, and repeating.   You will be doing the adjustments fairly quickly, using a screwdriver to electrically short the spark plug ADAPTER to the engine metal.  The engine must be warmed up a bit, even a garage type 3 minute warmup (for the INITIAL adjustment).   Then, thoroughly warmed-up, you do the final adjustments (a 10 mile ride will do).  The final adjustments take only a minute or three at the most, so you don't need a fan.  If you wished, for any garage-type adjustment, or an extended one, you could consider using a big fan, to prevent engine overheating.  This is especially so if you are doing the shorting method for your very first time which will usually take longer. Actually, this fan idea also applies to using method #1, the vacuum method.  While the complete synchronizing can be done with the fan, in your garage, & you will be quite close, the best final adjustments are done after a 10 mile+ ride.  

Your EARS & BRAIN will tell you all you need to know.  Your first time at this might take 15 minutes of playing until you get the idea.  Then, you can do it all in 5 minutes ...REALLY! 

You need adapters that absolutely, positively, without question!!!, will allow the existing spark plug caps to connect to these adapters & the adapters to then connect securely to the spark plug top threads.    These adapters must NOT!! flimsy.  That is, they MUST be such that there is ZERO chance of a spark plug or cap dis-connection. This means that whatever adapters you make/use, these adapters must NOT come loose.  If they come loose you could injure the coils in points models; or, coils and/or Hall device or module in the 1981 & later models. 

>>>Disregard ANYTHING you read in ANY book or on the internet, or ANYPLACE ELSE... that says to pull the spark plug caps off your engine to 'listen' or otherwise synchronize the carburetors.  That method is SAFE for ONLY the /2 or other early models (pre-1970) which use a magneto & have safety spark gaps built into the magneto (& such gaps have to be in good condition).   You can ruin or damage coils & electronics by failing to heed my advice here.   Certainly there is a shock hazard too. 

DO IT MY WAY!...    >>EXACTLY my way!<<

IF you chose a Method #1 meter or gauge, you don't short the plugs, nor need any adapters.   Some folks do the final adjustment with the shorting adapters after preliminary meter/gauge usage.  In SOME instances that is a NECESSITY...see the Hints section at the end of this article.   YOUR choice.

The shorting method IS MORE ACCURATE, as it takes into account ACTUAL differences, not just vacuum differences, between the cylinders.  It is dependent on YOU being careful & methodical.   Of course, this negates that nice feeling some folks seem to get by purchasing pricey test gear (that does not work as well, although SOME think their usage makes things easier).  The GROG HARMONIZER would be the best vacuum tool to use, if you want to do your synchronization and adjustment that way.    I like to do the idle MIXTURE adjustments by EAR.

The shorting adapters:
As I noted, you need something that fits securely at the spark plug top THREADED POST and also inside the spark plug cap.  The first thing that comes to mind is likely one of the removable small threaded barrels that many (NOT ALL) spark plugs come with, & that you have probably discarded.  Those do have the correct thread, obviously.  Some used to be made of BRASS, & those are preferred, as you can solder to them.  Most are now some sort of UNsolderable, but crimpable, aluminum alloy.  You can probably find some for free at a nearby auto repair place.  However, a spoke nipple, from spokes type wheel rims, will do very nicely, especially with its longer threaded area.

Into the spark plug CAP, you can insert any sort of properly fitting threaded rod as described a bit later.  If using one of the barrels of aluminum alloy for the spark plug threads, perhaps crimp a piece of 14 gauge house wiring solid copper inner conductor into it, leaving several threads so the barrel can screw onto the spark plug.  I made one of these up using, instead of rod or house wire, a few inch piece of old ignition wire, attached the adapter wire at the spark plug cap end by using the metal part from the top of an old spark plug; simply using a small hammer or vise to crush the old spark plug & remove the top innards. I had to fiddle with the wire to make it fit tightly enough into the spark plug cap (that the ignition wire fits into).  I don't recommend that method.  I don't bother carrying that, it looks unprofessional...although I admit to keeping a couple of especially crude looking ones in my Walus gauge box.    A very nice adaptor can be made with spoke material, see just below....and it is more secure!

The spark plug threads are 4 mm x 0.7 mm pitch.  You can, perhaps, find LONG screws or other threaded material at hardware stores, & nuts, if you wish to make your own version of these shorting adaptors from such items.

Here is a way to make a rather neat AND SECURE-FITTING adapter, using old spokes and spoke nipples:
The old wheels BMW spokes have the correct thread,  4 mm x 0.7 mm thread pitch.   If you have a wheel rebuilding store near you, obtain what you want from them, they are always throwing away old spokes and nipples.

Use the nipple (they are best) or spark plug screw top barrel adaptor.  Cut the spoke to 2 or 3 inchesin length; one end of which is the original spoke threaded end.  Put a TINY sharp notch on ONE SIDE of the non-threaded spoke end, close to the end (look into the spark plug electric cap for why and where the notch should go); then the spoke notch will engage with the stiff tiny wire inside the spark plug cap...where the spark plugs threads used to.  Explaining this another way:  the BMW or proper NGK spark plug caps have an internal cross-wire to 'grip' the spark plug threads.  The idea is to NOT let the adaptor jiggle off the spark plug cap during your synchronization efforts.

If you have a dual-plugged bike (two plugs per cylinder), you can make TWO spoke adapters for the top plugs, an ignition wire type adaptor for the bottom ones, & have the two places to do the shorting (each cylinder) very close to each other...maybe an inch or you can use a single screwdriver just like you would do single plug bike shorting.  Many ways to do this, including insulated parts in an insulated switching box with old-fashioned knife switches. SOME bikes, depending on tuning, etc., will allow you to continuously short BOTH bottom plugs to the cylinder head fins, when doing synchronization by shorting the TOP plugs.  On others, shorting the bottom plugs constantly reduces the idle speed too much...and you have to UNshort the bottom plugs anyway for final adjustments.  You have to THINK when shorting the bottom plugs this way, as you really need the RPM to be in the correct area for doing carburetor adjustments.  That is one of the reasons some folks have made insulated switch boxes with knife switches, to short both plugs on either cylinder a the same time, to engine ground. Others will use vacuum methods, instead of shorting methods (for dual plugged Airheads).

Be your own adapters/system of doing the shorting's.  Be sure all is secure in use!!

PHOTO:  An assortment of spark plug shorting & adapting tools.  These include the spoke & spoke nipple type. I have used all of these at one time or another, improvising on the spot at TechDays, etc.   The bungee cord (I actually used two of them) is here as I used the bungee to hold the bottom spark plug wire adaptors close to the upper plug shorting point....see text (yes, you'd then need TWO bungees).  Here is a source for a simple version of shorting tools; you may not find them listed on the website, so ask.



Here are some more spark plug adapters, two types.  Only the left two with the male threads are used on our Airheads:


How carburetors work (simplified explanation).  Vacuum-synchronization methods (this is not generally used with the above shorting tools).  Even if using shorting methods, read this:

A carburetor operates, as opposed to a fuel injector, by having an internal necked-down area through which incoming air passes, & that incoming air is speeded up by the squeezing effect of that neck.  Jakob Bernouli's theorem said that there is a DEcrease in pressure as velocity increases.  The decreased pressure, or 'vacuum', allows fuel to rise up in an opening/port/pipe ("sucked up") that has access to the fuel bowl.  The combination of the fluid gasoline & air rushing by the pipe or port opening turns into spray, hopefully well atomized into a gaseous mixture with the incoming air, but is more likely to have superfine droplets.  

There are places in the carburetor that fuel is sucked-up.  One is the central jet/needle assembly; another is a controlled size hole removable "idle jet", which then feeds one or more idle port holes located in the bottom throat area of the carburetor.  (I am avoiding here going into information that the enrichener on the CV carbs works, crudely, in a similar manner). The idle ports are always in the base of the carburetor on Airheads.  On the pure slide (NO butterfly valve) carburetors, the slide is mechanically coupled to the throttle, and the slide is adjustable by a screw when the throttle is turned OFF, so the slide is just the right amount above the idle port and at the same time allows some air for the VENTURI operation.  Adjustment of the slide height adjusts the idle RPM.  In the CV carburetor, the slide is not connected to the throttle, but there is a butterfly that is, and the butterfly closure point is adjustable for the idle RPM.   For both types of carburetors the throttle cable has its own adjustment, which is used to set the engine for equality between the cylinders when the throttle is SLIGHTLY opened ("slightly turned on"). 

The piston coming inwards (away from the cylinder head) in our Airheads on the INTAKE stroke (when the exhaust valve is closed, intake valve open); creates a partial vacuum in the cylinder, with reference to outside air.  When the butterfly valve (or 'slide' in pure slide carburetors) in a carburetor is closed, or nearly so (it can't be 100.000% closed as the engine needs some air to run), fuel & in some instances fuel/air mixture, is sucked out of the idle port(s) (holes) in the bottom of the carburetor body throat.  As the butterfly or slide is opened more, by you turning the throttle open (on the CV carbs, the butterfly is directly throttle controlled, but the slide is controlled by vacuum passageways inside the carburetor), the idle ports slowly stop functioning, depending on throttle opening amount, due to lowered vacuum at the idle ports....& now fuel comes out of the reasonably central jet area.  

The vacuum port for a vacuum gauge is simply a small hole, with an internal passageway into that venturi area.  The vacuum port has a small diameter pipe pressed into it, which is blocked off by a plug, screw, or connected to the other carburetor & a system for operating a smog device setup called Pulse Air on later models with rectangular air cleaners.    It is possible to sample vacuum after the carburetor, the readings will be of different effect with throttle opening, but it CAN be done, by a probe into the rubber hose, for instance.  I suggest that method not be done by the average person, as readings and use is somewhat different from that noted in this article.  The common method is to sample the vacuum at the carburetor throat as described. This is officially called Venturi Vacuum. 

For you nerdy folks: (1)  if the vacuum was sampled AFTER the carburetor, it is called, confusingly, Throttle Plate Vacuum; (2) IF the vacuum produced by the engine cylinder piston movement was a near perfect vacuum, the maximum vacuum that could be produced, in theory, is approximately 30 inches of mercury vacuum at sea level, where 30 inches of mercury is the standard accepted atmospheric pressure, but in practice it varies a bit with weather conditions.    30" of mercury is ~15 pounds per square inch.  As altitude increases, the air pressure around us generally goes down ~1 inch of mercury or half a pound per square inch, per thousand feet.  Some atmospheric effects can make small changes that are up or down with increasing altitude, but the over-all trend is ALWAYS down with increase in altitude.  Thus, at higher altitudes, there is less air pressure, which means more fuel tries to go through the carburetor per weight of air (it is air SPEED through the carburetor that causes the venturi vacuum), & the mixture thus gets richer. Richer means more fuel per air amount.   The CV ("constant velocity) carburetors tend to compensate SOME for rising altitude but total compensation is generally not possible.  On a practical basis, the cylinder vacuum is not going to be a perfect vacuum, but is going to be a few inches of mercury less. 

OK, nerdy section nearly over with:
As the throttle is opened, especially if suddenly opened & most especially with a directly operated mechanical slide (not a CV carburetor); the vacuum on the engine side of the carburetor greatly decreases, until engine rpm rises considerably from where it was when the throttle was suddenly opened.   If the throttle is very suddenly closed from a high RPM, there is very high vacuum at the carburetor output area, & the venturi has a fair amount as well, as the passageway is very small due to the closed throttle, air velocity being high.  Suddenly CLOSING a throttle on a carburetor, especially the slide (only) type, can cause a maximum-possible-fuel-flow through the idle ports.  But, if the throttle is OPENED suddenly, with generally low RPM, the idle ports might not be able to supply sufficient fuel, and the engine can stumble. 

Due to these effects, it is FAR better, FAR more sensitive, to adjust a carburetor's throttle cables not very far above idle; & conversely it is poorer, usually much so, to try to do any cable synchronization at quite high rpm.  You do NOT want to adjust the cable synchronization too close to idle rpm nor too high.   No need to get into all this any deeper.  I suggest you DISREGARD any books or literature, that say to set the throttle synchronization at 4000 rpm, for instance.    The best rpm to synchronize the CABLES (after you do the other adjustments FIRST!) is ~1300-1800 rpm; 1500 is a nice figure.  The BMW carburetors cable synchronization is done by adjusting the cable sheaths & this is ALWAYS adjusted LAST. During the earlier synchronization process, these throttle cables MUST have some SLACK at throttle-off position!!  There MUST be some slack AFTER you are done with all synchronization.  Usually it is ~1/8" or a slightly less.

Manometers, or any type of gauge or electronic method, either uses the vacuum in each carburetor venturi (separate manometers, or separate mercury sticks or separate gauges), or, reads the difference between the two carburetor venturis.  Either method can work fine, but differential methods are usually more sensitive, as they use electronics that can amplify results, & thereby are more accurate.

NO vacuum method is as good as my shorting method, which takes into account actual engine cylinder differences, and then is no need to purchase a vacuum synchronizer, vacuum gauge, manometer, etc.  Using shorting methods is nothing new, it has been published for many years, not only by me, but by various groups/clubs, etc.....especially in Europe.

IF you decide to purchase a vacuum measuring synchronization device, I recommend the Harmonizer, made by GROK....but you still may want to use your ear for idle adjustments.

Vacuum measuring devices usually incorporate a means, often called a SNUBBER, for dampening the otherwise jerky readings.  It is a very tiny restrictive hole or a porous material ... or other method that produces a small hole effect. Otherwise, the device's indication might pulsate rapidly or be much too sensitive & thereby be hard to read/average, as carburetor vacuum flow is actually a 'pulse train'.

Preliminary setup and discussion of cables & throttle system design:
Before you can do final synchronization, be sure of the following details.

ALWAYS be sure that the barrel fitting at the end of the cables, where they fit into the lever arms at the Bing CV type carburetors, have a bit of lubricant, heavy oil is OK; especially good is light moly grease on top of moly oil. I use moly oil because it gets into the barrel area better.  These barrel ends MUST be free to ROTATE.   Failure to have smoothly rotating barrels is a prime cause of frayed & broken cables (at the clutch lever at the bars too!) & lousy synchronization.

If your throttle unit at the bars is rough in its action, or the cables are improperly routed (tying them down strongly with extra tie-wraps is not a good idea), are going to be fighting yourself.  
There MUST be SOME free play in the outer jacket of both throttle cables, throttle at the bars being off (hands off the throttle). SLIGHTLY LIFT the outer cable sheath at the carburetors, about 1/8" of free outer sheath play is reasonable.

BMW throttle cables  are NOT to be lubricated.   Genuine BMW cables are internally LINED with a Nylon or Teflon-like substance. Original types, if any still exist, pre-1978, CAN be lubricated as a last resort before replacement). Cables DO wear out & should be replaced when they are acting up, getting stiff, etc.   Besides their lining wearing (which it tends to do at bends), the steel strands might fray, & one or more break.  This is fairly common at the throttle levers end of the cable at the CV carburetors, usually due to a lack of lubricant at the barrel fitting, as the barrel must move a considerable amount in rotation, and may not be smooth in doing that (in which case that needs fixing and then lubrication).  It is also a rather substantial problem from folks bending the throttle cable on the left carburetor while checking the oil dipstick.    ONE broken strand at the carburetor lever area CAN lead to total failure ...I have seen this within a FEW hundred miles of the first strand breaking.  A different cable problem happens rarely, a tip disconnects from the cable due to faulty manufacture, or, excessive pulling due to friction (wherever). 

Preliminary eyeball mechanical synchronization has been done??  If your carburetors have been synchronized before, & you have not messed with adjustments in a major way, you need not do this step, unless you want to be SURE.  I ALWAYS do these eyeball checks.

BMW has TWO types of basic throttle cable designs, one design has a single throttle cable at the handlebars; the other design has TWO cables at the bars throttle.  The single type is more stable, less affected by turning the bars. The single cable type uses a round T junction tubular part located under the fuel tank.  That T-junction has one adjustment: for top cable length, for SOME free-play.  It is not adjusted for carburetor cables free-play.  You only need to adjust that top cable adjustor so that there is a SMALL amount of free play in the upper cable AND to ensure that moving the handlebars from side to side has no effect on carburetion.  Of course that is true of the bottom cables too.

Before you do carburetor synchronization, the upper throttle cable(s), assuming you have properly synchronized the gears & greased them not too long ago...will have a small amount of free play. Be sure this is true.  You can NOT properly synchronize the carburetors unless the throttle cable(s) at the handlebars has SOME free play. The SINGLE cable model free play can be seen by rotating the throttle slightly or pulling on the cable sheath...., it is better to pull, slightly, on the single throttle cable,  as it enters the throttle assembly.  About 3/32" is about right.   DO NOT try to rotate the throttle further off for this, it needs only to be in its natural off position (any friction device is turned OFF).  SOME free play must exist throughout the turning of the bars, from side to side.  If the free play is excessive, throttle feel will not only be sloppy off-idle, but you MAY not be able to REACH full throttle, no matter how far you twist the grip. That is something I check when doing the final throttle cable adjustments AT the carburetors.     

I'll say that a bit differently:  The free play adjustment is in more than one place.  The single throttle cable models have the upper cable free play set at the nose of the T adaptor under the tank as I noted, above. There is supposed to be a rubber weatherproofing cover there. If the motorcycle has TWO cables to the throttle grip area, the free play adjustment IS ONLY AT THE CARBS via the throttle sheath adjustment.   ALL Airheads, one or two cables at the handlebars, have the carburetor cables free play (and, throttle CABLES synchronization) adjustable AT the carburetors.  Turning the bars considerably can move the top cable(s).  Thus, the ONE cable model is likely to keep the carburetor throttle cables actually AT your synchronization of the throttle cables, better than two cable versions.   TOP throttle cable (ONE cable at handlebars models ONLY) have an adjustment at the tubular connector under the fuel tank (which is NOT present on two handlebar cable models).  Once set, it is NOT part of the synchronization, and, the setting is likely to remain good for YEARS. 

If throttle INTERNALS at the handlebars are improperly assembled (there are synchronizing mechanical marks to line up INternally... you can see that with the one large screw removed and cover off), you will likely be UNABLE to get full or proper throttle operation.  Rotation will be limited.  THUS, in working with the cables & carburetors, lubricating & proper re-assembly of the throttle parts at the bars is to be done FIRST...BEFORE adjusting carburetor synchronization....unless already OK.

NERDY: the throttle CAM is not the same on 32 mm & 40 mm carburetors.

WHY the complicated throttle assembly for the throttle cables?  Cam? Chain? Gears?    .....
The BMW throttle design gives a very smooth straight-pull on the cable(s) at the throttle.  It gives a SLOWER proportional carburetor throttle effect at low settings, & as the throttle is opened, the effect increases non-linearly.....a very nice design, due to the CAM shape. This makes for a throttle that is not just smooth, but has PROGRESSIVE effects, allowing for much more accuracy for small throttle movements at just above and to maybe 1/3 or 1/2 throttle; you can also take this to mean less engine jerkiness from trying to control small throttle movements.   It is too bad that this sort of design, magnified some, is not used on the Japanese and other "crotch rockets".

/5 models:  If you have /5 CV carburetors, these have the return springs AROUND the throttle cable sheaths at the carburetors, & you MAY find the throttle action very stiff & probably has too much return spring force.  First you should check the cables for stiff operation for any reason.  If the cables are OK you may want to modify the spring setup.  Simply reducing the spring force will help some, but if you go too far, the throttles won't return fully.   You may want to fashion your own conversion, & looking at later carburetors will give you ideas.  If you CHANGE to later carburetors, the problem is solved easily, if often expensively.  The R75/7 carburetors are GREAT for the /5.

Just what to do to the original stock carburetors to reduce throttle force has stumped more than a few owners, since the tops of the /5 CV carbs are not all that easy to figure out how to add the later style of spring, which reduces hand force at the bars GREATLY.  HERE's a method you MIGHT LIKE:  Add a LARGE washer at each carburetor in the threaded area of the throttle fastening to the carburetor.  This is a bit tricky to do neatly, & one of these days I will do another /5 & post a photo here.  You drill a tiny hole in that washer; the spring upper mount then becomes the washer, using the later type of spring.  Take a look at a later carburetor that has the spring connected to the carburetor body & the levers on the carburetor shaft.

Except for the /5 models in UNmodified condition, lighter CV carburetor springs are available....they are not the same for 32 and 40 mm carburetors. Check with your dealership; or, Ted Porter's Beemershop in California.  There is a type 606
for the 32 mm carburetors that have 3 digit model numbers; a type 908 for the earlier 2 digit 32 mm types, & a type 312 for the 40 mm carbs.  These will give lighter feeling to the throttle, yet are still adequate to return the throttle to idle.  The throttle assembly might last longer too.  These are NOT for the UNmodified /5 carburetors.


This could be called a STATIC synchronization; that is, the motor is OFF & no instruments in use.

If the carburetor adjustments were all changed radically, perhaps you overhauled the carburetors, changed cables, whatever; you SHOULD do a PRELIMINARY CABLE synchronization by eyeball.   If this is done accurately enough, it is usually good enough to allow you to go riding to warm up the engine ....until you can do it properly with tools or instruments!!!!  I will ASSUME here that you have the idle mixture and idle stop screws set for the preliminary values, as seen in literature.

1.  If you have the SINGLE CABLE at the throttle at the handlebars, adjust that cable so it has a small amount of free play, perhaps 1/8", throttle off. If two cables there, disregard.

2   Adjust the throttle cables lengths at the carburetors so the handlebar throttle just begins to lift the carburetors levers (or slide) at the exact same time as you BARELY increase the throttle, from idle position, a very tiny amount. #1 eyeball works fine.   Leave the cables with maybe not quite 1/8" of slack in their outer cable sheath at throttle off, but hand off the throttle.  You will need to play with the idea awhile, as it is not the same view to you, left and right carburetors.  

If you are NOT doing JUST an eyeball throttle cable adjustment, that is, you are ready to do a real synchronization, proceed as follows:

Make sure the two carburetors have close to the same small amount of idle stop adjustment...this is beyond the butterflies being totally closed (also adjust the stops equally for a wee bit of lever off the stop, perhaps 1-1/2 turns).   The slide only carburetors are a bit different, but with the same idea.  If you played with the idle mixture adjustment, set it per whatever book you have, as far as turns outward, from a GENTLE inward stop.  This will be 1/2 to 1-1/2 turns PER THE BOOK, so check for YOUR carburetor, probably the BING book or other literature.  You will also want the choke (enrichener) cables (if you have a CV carburetor) adjusted for full off with their levers on the carburetor stops, and also able to reach approximately or nearly the full-on stops at the same time as you full engage the enrichener lever at the bars, usually marked CHOKE.   There is no official synchronizing of the choke cables beyond this.   If you have T units under the tank, they have adjustments to take up excessive slack for the upper throttle & choke cables, but a wee bit of slack is needed, & turning the bars back and forth should NOT change the slack enough to remove it all.  For those NOT having the T units under the tank, you have TWO throttle cables, which go directly from carburetors to throttle at the bars.  With the throttle OFF, hand NOT rotating the throttle backwards or forwards from idle resting position, there MUST BE approximately the same amount of slack in the outer jacket at the carbs (lift the outer jacket slightly to see this).   Near but not over 1/8" is correct.

Assuming that you DO have some throttle outer cable slack at the carburetors & at the bars cable too if you have just one, now you are ready to get your tools, fan, whatever, all laid out, ready for your return.  I said RETURN, because you should go for a ride..... that is, if you are doing a complete, rather than eyeball adjustment of the cables (which you may have already just done).

HINT:   A goodly sized fan is needed, AFTER your warm-up ride, if you take more than a FEW minutes to do the adjustments.  Here is a way to get an EXCELLENT fan for basically nothing!  You do not absolutely need this, but it sure is nice!:::

Go to one or more local heating contractors, until you find a cooperative one ("sure, you can have an old squirrel cage motor, for free").   Heating/cooling contractors are always removing old home heaters & replacing them.  MOST of these heaters contain a perfectly good motor, that has shafts running out both ends, & the shafts have mounted to them squirrel-cage type rotary blade fans.  You want the type with a squirrel cage at both ends.   There is a surrounding metal shrouding that you want is all one assembly.  Most of these motors are heavy duty capacitor-start types, with plug tap selection for speeds.   Adapt a power cord & maybe make up some sort of simple wood piece to keep your new FAN from rolling about on your garage floor & maybe to aim the fan slightly upward, have a dual output high volume fan!  It is not a bad idea to put some chicken wire over the intake ends to prevent things in your shop, like rags, or your dog/cat, from getting into the fans.   The lowest speed setting is usually the correct one.   You will find this fan also very nice to use after a ride to cool the engine (and exhaust pipes!) rapidly for other work.  You may even want to use it before covering your bike.  Go for a 10 mile or longer ride.   1 or 2 miles is NOT enough!  A garage warmup is NOT adequate for the FULL procedure!   If you are forced to warm the bike up in the garage, the entire engine should be warm, & you MUST use that dual squirrel-cage fan, otherwise the engine will definitely overheat.  Warmup at about 1400 rpm is just fine.  A ride is better!   The fan needs to be in front of the front wheel, not behind the rear wheel.

Immediately upon your return, put the bike on its center stand, do not put it on the side-stand first.  This will avoid any fuel imbalances in the carburetors.   Leave the engine running or restart it when you begin.  Position the fan, turn it on, it should be in front of the front wheel, blowing over the cylinders.  Have the fan outlets tilted slightly upward for that, if need-be.  Do not overcool the cylinders & carburetors.  If you are experienced, you will be quick about doing synchronization, and likely NOT need a fan.

You are going to do a dynamic synchronization.  Once the carburetors are fully & properly adjusted, future synchronizations are likely to be FAR easier with FAR less effort, & take very little time; and NO fan needed!   NOTE that the idle rpm adjustments, & to some reasonable extent the idle mixture adjustments, tend to be rather stable over a long period of time/mileage, & in the future, while you may occasionally have to make an adjustment, usually you will be adjusting only the cable lengths.,,,and even then, only ONE; less often will you be adjusting idle mixtures & idle stops.   With good cables & the rest of the items previously mentioned all in good condition, 5000 mile intervals are all that is needed for quick synchronization. 

It is NORMAL for an Airhead to idle more slowly if the engine is not FULLY warmed up.  Your airhead will ALSO idle at a slower rpm as you ride to a different elevation.  Adjustment to help compensate for a too low or too high idle for altitude changes you think you will do, can be thought of & compensated somewhat for, as you do your dynamic synchronization.  That is an advanced thing to do. I will describe it in NERDY, below.

DYNAMIC SYNCHRONIZATION:   Because several methods could be used, and because you might have slide-only carburetors (that means NON-CV, even though CV carburetors DO have slides), I will describe the BASIC & SIMPLE method first, because it is common to all.  I will assume a vacuum operated meter or gauge or manometer, and not the spark plug shorting method.

1A.  Adjust the idle stop screws for a balanced gauge/etc. readings.  If the rpm is too high,  set each idle stop screw a wee bit less; or a wee bit more if idle is too low, and then go back & adjust one of them for balance.  You want to end up with a balanced indication on your test device, & a rpm of about 900-1000 for bikes before the 1981 changeover to the light clutch carrier (instead of the older heavy flywheel).   For the 1981 & later, & any year with dual-plugging conversion, I recommend a target of 1025 rpm and up to 1200 is OK.   If your tach is reasonably accurate, you may use it.   NOTE that excessively slow idle rpm will result in poor oiling of the timing chain & timing sprockets, & higher sensitivity to miss-adjustments.  If you want to, for any and all Airheads, just use 1050 RPM.

1B.  NERDY:   You could, you do not have to, select a FINAL idle rpm & idle mixture FINAL setting SOMEWHAT dependent on WHERE you intend to ride, & where the adjustment is being done in the first place.  This is because idle rpm tends to DEcrease with INcreasing altitude.   Thus, if you ride exclusively at sea level to perhaps about 3000 feet or so, I would target 1050 rpm.  If you were doing the synchronization AT a higher altitude...say 6000 feet or higher (and especially if on a quite warm day)... then I would set the idle for maybe 900 rpm, so the idle rpm will not increase too much when going down in altitude.  Once you get a bit experienced at this, you will realize that the IDLE MIXTURES are also changing with altitude, & you can make a SMALL change to them.  Those adjusting AT high altitudes might want to make the idle mixture screw a WEE bit more RICHER;  sea level adjustment folks use a WEE bit leaner.  The Bing CV carbs mixture screws turn INwards for leaner, & the slide carburetors turn OUTwards for leaner.    We are talking about 1/4th turn maximum difference here!  If you are a beginner at adjusting carburetors, I suggest you IGNORE the two paragraphs in this NERDY section....because my normal procedure is to have you set the idle SLIGHTLY rich....but, try this nerdy section if you want to, it does give better results.

2.  Now that you have the proper idle rpm, & balanced on the meter/gauge/whatever, you need to adjust the idle mixture.  You WILL need to blip the throttle now & then to clear the engine of any fuel loading-up, just prior to ANY adjustments, idle, mixture, cables.  After blipping, allow a few seconds for engine stabilization.   Start with ONE carburetor.  The idle mixture screw is adjusted BY EAR, & adjusted very slowly, perhaps initially a 1/4th turn over 5 seconds. As you approach the sweet spot, adjust 1/8th of a turn at a time, until the engine sounds the smoothest, & the idle speed the highest.  Then adjust RICHER by 1/10th or 1/8th turn.  This will be explained further.

IMPORTANT!     You may have to start with the idle mixture screw outwards considerably more than you started with, as you may not know what the last person set the screw at, ....and/or... the factory initial adjustment recommendation could be too far in to begin with.
   It is safe to start 2 full turns outward from a gentle gentle seating, thus you won't mistake the proper setting from starting too far inwards to begin with (trying to move it even further in, & NOT finding the correct adjustment.  I seldom ever use the recommended initial idle screw adjustment. I just use 1-1/2 turns or bit more outwards to begin with.  I suggest you practice, the first time, with adjusting the mixture screw 2+ turns outwards to start with; then you will easily see what proper adjustment means, as you screw it inwards.

THEN do the other carburetor.  If there was any rpm change at the end of any of this, go back & readjust BOTH idle rpm (throttle stop adjustment) AND its balance.  You may well have to repeat this process of mixture adjustment & idle stop screws SEVERAL times in the beginning.  Take your time.   What you want to end up with is the idle mixture screw in the best position for smoothness & highest idle rpm, & a balanced indication on your meter....and the proper rpm; AND ~1/8th turn more RICH (unless doing the Nerdy type adjustment, which is slightly different).   The idle stop screw & mixture screws interact with each other!  If things are WAY out of correctness, they interact mightily!    Blip the throttle occasionally, be sure the engine settles down before doing another wee adjustment & reading.   

You will find, on the Bing CV carburetors especially, that the idle mixture screw is very sensitive in the INwards direction, & far less sensitive in the OUTward direction, especially when you are close to the correct adjustment.

I suggest that you leave the mixture screw in the middle of the smallish adjustment range that causes the engine to SLOW or even stumble a bit if inwards too much, & slow a bit if outward too much. Another way to state this: I suggest you leave the idle MIXTURE screw on the Bing CV a bit OUTwards from optimum rpm/sound (maybe 1/8th turn), and about the same amount INwards on the slide-only carburetors. This results in slight richness.

You want to end up with the idle mixtures correct and slightly rich, the idle rpm correct, the idle balance indication correct.  Do NOT make the mistake of starting this procedure from the idle mixture screw too far inwards, & then rotating it farther inwards.  It is always better to start with it too far outwards.  Once done correctly, you need make only SMALL changes in the future (LESS THAN 1/4th a turn each way, to find the proper position).  NOTE!....there MUST BE throttle cable sheath slack!

YOU ARE NOW DONE WITH THE IDLE ADJUSTMENTS!  Future adjustments will be very quick.

3.  Adjusting the cable lengths (this is the last adjustment after any others):
This is BEST done at a modestly low rpm, an rpm above the idle rpm, but not too far above.  The farther above the idle rpm, the less sensitive the adjustments & the harder it is to then get it right for the more critical area, the 'just-off-idle' transition position.  Because of this, I recommend 1300-1800 rpm (I suggest using 1500).  You can lock the throttle if you have a friction screw or something else at the bars, or, just hold the throttle, use a rubber band, whatever. 

What you will do is to rotate the throttle for any specific rpm in the range of 1300-1800 (whatever rpm you want in that range), hold/fix the throttle, & then look at your balancing device.  If not at balance, change the length of ONE of the throttle cables & try again. You can get a feel for what direction, by lifting the outer sheath on one cable, slightly.  You want to end up with a balanced readout, AND, a small amount of cable slack at throttle OFF (carburetor levers must be on stops at the carburetors)...3/32" is fine.... not critical though.  Do not have so much slack that the outer sheath ferrule could come out of the receptacle in the carburetor fitting, nor so little that turning the handlebars would cause the idle to be affected by the cable length.   

Finer points:   
a.  Raise the rpm a fair amount & see if the balance still holds relatively well.  If it does not, you might have a diaphragm or leak problem. b.  
When blipping the throttle; or, suddenly and quickly moving the throttle to OFF from perhaps 4000 rpm...or whatever....& you see largish equal or unequal vacuum changes, SOME of that is due to unequal springs on top of the CV slides (if your model has springs), or differences in diaphragms.....etc.   All Airheads will have SOME such vacuum changes....less is better.... & unless are advised to leave things alone. 

YOU ARE NOW DONE!  You should now have a bike that starts well, warms up to a stable idle fairly fast, has good throttle feel 'coming off idle', ETC.

The shorting-of-spark-plugs method:

You simply will be modifying the previous methods, and you won't be using gauges. 

Here is the procedure for doing it with the shorting device tools you made or purchased:

1.  The two screwdrivers with the plastic insulated handles ready? The wrench for the throttle cable nut?  Fan?  Screwdriver to adjust the carbs?

2.  In your garage for the preliminary procedure....OR AFTER you put the bike on the center-stand following your 10 mile+ warmup ride:
WITH ENGINE OFF, IGNITION OFF, pull the spark plug caps off.  Install your spark plugs/caps shorting devices.  Be sure it is all secure; it is critical that the spark plug shorting adaptors can NOT fall off the spark plugs, the cap fly off, etc.   Having the system become an open-circuit with the engine running will damage the ignition. 

3.  You now start the engine.  Use the fan if you need to (you need to if your adjustment period goes beyond a few minutes).  

4.  Instead of a meter or other device showing you the cylinder balancing, you simply will short one spark plug adapter bare wire to a nearby fin; then remove that screwdriver induced short, & short the other spark plug adapter wire...back & forth.   You are killing one cylinder at a time.
5.  First, you will do the IDLE. You will, with one insulated screwdriver, short out one cylinder for ~3 actual seconds; then remove that screwdriver short and move the screwdriver to the other cylinder to short out the other cylinder.   You will make the same as earlier described adjustment of the idle stops for rpm & balance...balance here being the SAME SOUND/RPM from one cylinder & the other cylinder.  You also make the same mixture adjustment as previously described, & that MIXTURE adjustment, as in the gauge method, is done with NO shorting of the plugs.    Blip the throttle now & then to clear the engine, then wait a few seconds for the engine to settle down.

You will have the same back & forth idle stops & mixtures to do, until it is all OK & no further improvement can be made.

You want to end up with proper rpm & proper balance, at idle rpm, throttle off.
e sure you have some slack in the throttle cables, throttle off.

6.  With the idle adjustments done....YOU MUST NOW synchronize the cables.  This is a bit different in a couple of ways.  There are TWO cable synchronizations to watch for.  First one is a coarse adjustment.  Both are done at the same 1300-1800 rpm (YOU pick the rpm you like, but I recommend ~1500).

Short the plugs as before, left, then right, then left...listening to the engine.  Each time one plug is shorted, listen.  As with the idle adjustments, both plugs are NEVER shorted at the same time (one plug per cylinder meant here, not a dual-plugged bike).  Allow about 3 or 4 seconds of shorting time for EACH plug.  Adjust one, or both cable lengths, as required for balance...the same sound.  Remember not to run out of cable adjustment...cables must have slack at throttle OFF.  Remember to clear the engine if it needs it.

The final adjustment is the same, short the plug circuit, as before, left, then right, then left, listening to the engine, BUT....this time allowing only ~1 second of shorting time.  You might need a tiny cable adjustment.

In most all of the shorting method work, you are NOT looking at the tach, you are LISTENING to the engine.  The only time to look at the tach is to be sure the ending idle rpm, & the rpm during CABLE adjustment, are both at approximately the correct rpm.

This method will result in a VERY smooth engine.


Dual plugging conversions:

If you want to use the shorting method, it is sometimes better to short out BOTH plugs on one side at the same time, and then alternate to the other side, similarly both shorted out.....back and forth.  This can be mechanically fun & games unless you make a special switch or device to do this.  I have made an extra long and stiff shorting wire to allow it to be done with a single screwdriver in each hand. HOWEVER....This method, on SOME DUAL PLUGGED bikes, may reduce the idle rpm so far that the engine stops, which you do not want.  The fix for that is to shorting out both cylinder's bottom plugs, leaving them both fully shorted-out during the procedure, which usually works OK, & is my recommended method if you have engine stalling problems.  If you use this method, you need two extra shorting adapters, and two each short jumper wires with alligator clips on each of the 4 wire ends.  You clip the alligators to a fin and to the adapter....and, of course, the spark plug wire fits onto the adapter!

There is sometimes confusion on how the coils are to be wired/connected, on a dual-plug installation.  The PROPER setup for a dual-plug installation is for ONE plug of EACH cylinder fired by ONE of the  dual-output coils....and to a lesser importance you can also do it so the top plugs are fired by one coil, the bottom plugs by another coil.   AVOID using ONE of the twin-tower coils to only fire one cylinder, and the other twin-tower coil firing the other cylinder....multiple reasons, but the ignition is much poorer.

Most folks probably do synchronization of the dual-plugged engines with vacuum methods; but, see hints above;  and the below items.

Hints for all types of carburetors, and all types of synchronization methods:

Even with clean idle passages & everything in good condition, irregular idle & strange symptoms at idle & off-idle, & ESPECIALLY irregular idle rpm, are often due to a bad idle mixture adjustment and/or idle jet rubber O-rings.  This can also happen on CV carburetors withOUT the springs above the slides, the purpose of which is to ensure positive seating of the slide at the bottom at idle, & to smoothly return the slide downwards.  

If you have a R75/5 with original early number carburetors that is particularly difficult to start or has very irregular idling & everything else checks out fine, be SURE to check the slides, to be SURE they are BOTTOMING fully & not hung up slightly.  This can happen with other airheads, but MUCH less likely.
The earliest R75/5 carburetors had many problems & a completely separate article is on this website for those, although synchronization is same:

Springs can be added to dome-type carbs: see the two Bing CV articles on this website. 

Be SURE there is throttle cable slack at the carbs!!!!

It's CRITICAL that the enrichener parts be not mixed up left to right, & are installed correctly.  It is also very important that the butterfly valves on the CV carbs be installed correctly.  Some enricheners were shipped by Bing with WRONG markings.  See my two BING CV carburetor articles for more information than you can believe....AND photos:

On the /5 models, the intake plastic tubes have slots at the carburetor end, & can drip breather condensate oil on your foot.  A few had the slots too deep, & the following does not work, but for most, put the band clamp adjustor at the carburetor INlet at the TOP...which will stop the oil dripping.

Once in awhile there will be an Airhead with the CV carbs that will seemingly adjust just fine with the vacuum gauge method....for idle mixture, idle rpm & off-idle for the cables... & yet will not idle smoothly.   Sometimes the owner will go crazy trying to find the reason, & check ignition points (if present), timing, valve clearance, compression pressure, spark plugs & their caps, cables, coils, ETC.    Nothing is found wrong, except the poor idle.   The problem is then thought to be inside the carburetors (which it is!)...but, what?......well, it can be butterfly's ...improperly installed.  See photos in my above carb articles for the butterfly installation. THIS IS CRITICAL! I have even seen a new carburetor that had the butterfly in backwards!

Note, AGAIN, that the factory, OR SOMEONE ELSE, may have installed the enrichener parts backwards, AND note that some, even late models!... are MISS-MARKED.  See my other carburetor articles for PHOTOS of the correct...and wrong installation.

Finally, some nasty effects, that make you think it is the carburetors or even ignition, can happen if the KILL switch is corroded enough internally.  Bypass it for proof.

04/22/2003:  add .htm title; clarifications
07/31/2003:  add note at top area on lubing cables; minor editing for clarity in the entire body of article, add dual plug info.
09/21/2003:  start a new section..wee hints; go over entire article and clarify details here and there.
09/26/2003:  add picture of spark plug adapter tools
03/08/2004:  add hint 2
10/03/2004:  syntax and grammar here and there; plus updated information on dual plugging & synchronization, and revised the Hints section, adding more information.
03/27/2005:  add 5.
11/03/2006:   Update entire article for emphasis and clarity.
07/06/2011:  clarify idle mixture and rpm compensation methods
11/14/2011:  slight clarifications in dual-plugging comments
11/15/2011:  wee bit more clarification
04/27/2012:  Edit for clarity
08/08/2012:  Expand and update cable maintenance and replacement section
10/14/2012:  Add QR code, add language button, update Google Ad-Sense code
12/13/2012:  Go over article, make minor improvements in clarity.
In 2013, remove language button javascript code, it was causing some browsers to have problems.
06/01/2014:  Recheck article.
08/03/2014:  minor changes for clarity, section separation
08/25/2015:  Cleanup
09/14/2015:  Make slight changes to recommendations for synch tools.
10/04/2015:  Clarify vacuum and shorting methods, add link for shorting tools.
12/06/2015:  Clean up article, moving things to left and shortening right side.  Meta-codes work.  Some clarifications.
12/17/2015:  Meta-codes.  Clean up article and left justify for smaller screens.
04/01/2016:  Update metacodes, layout, redundancies, colors, fonts, clarifications. 
04/25/2016:  Add note re: Grok's death; add new photo of solid brass spark plug adapters. 
09/13/2016:  Update Grok family information.

Copyright, 2014, R. Fleischer

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Last check/edit: Tuesday, September 13, 2016