Carburetors on BMW Airhead Motorcycles; how to properly synchronize them
© 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.
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 may
have been 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, but on-condition.
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
If someone makes a habit of bending the left throttle cable at the carburetor while removing
& replacing the 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!
A BIG reason for cable problems is failure to lubricate the barrels at the carbs; I do
it after EVERY wash job, after drying. A moly-type oil/grease is best.
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
grease or moly oil.
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!
>>>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 theirs?
(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 in 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
even near 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, which produces a situation called 'a rocking couple'. That is actual
terminology; simply said, it 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), this affects cam timing & ignition. While the stock ignition
is easily adjusted to mostly compensate, 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. While compensated-for by adjusting the ignition timing, that does NOT
compensate for the retarding of the camshaft.
External to the engine core are variations in 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 is 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 in what was described above; and, maybe even diaphragm differences in
the carburetors (I could make a long list of things having effects)....will cause engine
roughness or vibration, often 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'....not to mention an unstable idle, perhaps a large increase in idle rpm after
full warm-up; perhaps being hard to start, act very lean, etc. If the idle mixture is
improperly set, it is not unusual to see red exhaust pipes near the heads....& other things,
including a large change, usually UPwards, as the engine warms fully.
NOTE: 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.
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 strip...you
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
I see a fair amount of Airheads that do not have sufficient free-play in the throttle
cables (sometimes choke cables too). If the needed throttle free-play needed is
not there, 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.
It is beyond the scope of this article...which is already lengthy....to 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 simple tools, & 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?
You will need a single 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 very fast.
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. I believe it is better & much more
useful than the TwinMax, & more useful for most folks than
the Walus (which are hard-to-find anyway, having not been
made in decades). (Snowbum has & uses a Walus, FYI).
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 the Airheads. Note that early CV carburetors that DID have vacuum
ports (NOT all did) had the ports sticking out from the carburetor bosses horizontally.
All later models had ports, & they stick downwards from the underside, these later ones
usually 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 conversion 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 MUCH 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. 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: http://www.northwoodsairheads.com/.
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...
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 run the engine at idle... & also just
off-idle (above idle a small amount), ....& make adjustments to the carburetors, while
you run the engine first 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 to 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, you could consider using a big fan,
to prevent engine overheating. This is especially so if you are doing the shorting method
for the 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 OR
MUCH LESS (NOT kidding!).
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!!...be 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 (anything 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 above is a very wordy section. In actual practice, it is VERY simple.
The shorting adapters:
You need something that fits securely at the spark plug top THREADED POST. 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 probably discarded for your Airhead.
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. Into the spark plug CAP, you can insert
any sort of properly fitting threaded rod as described a bit later. If using one of these
barrels of aluminum alloy, perhaps crimp a piece of 14 gauge house wiring solid copper
inner conductor to it, the other end of the barrel screwing 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 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.
The threads are 4 mm x 0.7 mm pitch. You can, perhaps, find screws at hardware
stores, & nuts, if you wish to make your own version of these shorting adaptors from
Here is a way to make a rather neat adapter, using an old spoke nipple.
BMW spokes have the correct thread, 4 x 0.7 mm. Use that nipple & the spoke
that it fits, & cut the spoke to a couple inches or bit more in length;...put a teeny notch
on the SIDE of the upper end; then the spoke will be pretty solid in the spark plug cap,
which has a cross-wire to 'grip' the spark plug threads. 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 so....so 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.
Be creative...design 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 the shorting tools:
Theory: vacuum-synchronization methods (this is not generally
used with the above shorting tools). Even if using shorting
methods, read this:
A carburetor, as opposed to a fuel injector, works by having a necked-down area through
which incoming air passes, & that incoming air is speeded up by the 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 some opening/port/pipe
(sucked up if you want to think of it that way) that has access to the fuel bowl, & turns itself
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 carburetors that fuel is sucked-up.
One is the central jet/needle assembly; another is a passageway to an idle jet, which then
feeds one or more very teensy-tiny 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 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,
with reference to outside air. When the butterfly valve (or 'slide' in pure slide carburetors) i
n 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 stop
working due to lowered vacuum at the idle ports....& fuel comes out of the central jet area.
The vacuum port for a gauge is simply a small hole, with an internal passageway into that
venturi area, sampling the lowered pressure there, & that hole or enlarged entrance of it,
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 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.
The common method is to sample the vacuum at the carburetor throat as described. This is
officially called Venturi Vacuum. For you nerdy folks, if the vacuum was sampled AFTER
the carburetor, it is called, confusingly, Throttle Plate Vacuum.
IF the vacuum produced by the engine cylinder piston movement was a perfect vacuum,
that is, total 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, & the mixture thus gets
richer. 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.
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 a 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,
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 too 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 slight bit less.
Manometers, or any type of gauge or electronic method, either measures the vacuum in
each carburetor venturi (separate manometers, or separate mercury sticks or separate
gauges), or, reads the difference between the two carburetors. Either method can work
fine, but differential methods are usually more sensitive & thereby more accurate.
NO vacuum method is as good as my shorting method, which takes into account
actual engine cylinder differences...and...there is then no need to purchase a
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 am recommending the Harmonizer, made by GROK.
Vacuum measuring devices may incorporate a dampening means, often called a snubber,
a very tiny restrictive hole or porous material or 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 and system design:
Before you can do final synchronization, be sure of the following:
Preliminary eyeball synchronization has been done. If your carburetors have been
synchronized before, & you have not messed with anything on the carburetors, you
need not do this step, unless you want to be SURE. I ALWAYS do these eyeball checks.
See below for how to do the preliminary eyeball synchronization.
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 fine on
them, especially good is light moly grease or moly oil. 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. For the
LEFT cable, many will damage the cable by bending it when checking oil level with the
Cable condition internally, see below, is important. 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), etc....you 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 (original types, if any still exist, pre-1978, CAN be lubricated) are
NOT to be lubricated, except as a last resort before you can get new ones. They are
LINED with a Nylon or Teflon-like substance. 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 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. It is also a problem from folks bending the throttle cable on the left
carburetor, while checking the oil dipstick. ONE broken strand at the carburetor
lever CAN lead to total failure ...I have seen this within a FEW hundred miles.
A different cable problem happens rarely, a tip disconnects from the cable.
BMW has TWO types of basic throttle cable designs, one design has a single
cable at the throttle at the bars; 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 to be 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.
When synchronization is completed the upper throttle cable(s), assuming you
have properly synchronized the gears & greased them not too long ago...and
have adjusted things properly, will have a small amount of free play. Be sure
this is true. The SINGLE cable model free play can be seen by rotating the
throttle slightly or pulling on the cable...., it is better to pull, slightly, on the
single throttle cable, as it enters the throttle assembly. About 3/32" is about
right. For this single cable model, as noted previously there is an under tank
T-junction adjustment. DO NOT try to rotate the throttle further off for this
check. DO NOT have any friction knob adjusted for any friction for this check.
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 another final thing to check.
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
adjustable AT the carburetors. Turning the bars considerably makes a
considerable amount of movement on 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, thus 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!!!!
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; you can also take this to ALSO mean less engine jerkiness.
/5 models: If you have early /5 CV carburetors, the type with the return springs AROUND
the throttle cable sheaths at the carburetors, & you find the throttle action very stiff & probably
has too much return spring force, & you have checked the cables for stiff operation, you may
want to modify the spring setup. Simply reducing the spring force will help some, & if you go
too far, the throttles won't return properly, so do not. You may want to fashion your own
conversion, & looking at other carburetors will give you ideas. If you CHANGE to later
carburetors/parts, the problem is solved easily, if expensively. The R75/7 carburetors
are GREAT for the /5. Just what to fashion, or do, 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
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.
PRELIMINARY "EYEBALL" CABLE SYNCHRONIZATION:
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
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 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,
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 MUST go for a ride..... that is, if you are
doing a complete, rather than eyeball adjustment of the cables.
HINT: A goodly sized fan is needed, AFTER your warm-up ride, if you take more than a
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 too...it 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....and....you 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 flying 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! If you are forced to warm the bike up in the garage, the entire engine must 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....MUCH 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. 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 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.
1. 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. 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 mis-adjustments. If you want to, for any and all Airheads, just use 1025 RPM.
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 vary
with altitude. Thus, if you ride exclusively at sea level to perhaps about 2500 feet
or so, I would target 950 rpm for pre-1981. If you were doing the synchronization
AT a higher altitude...say 6000 feet or higher... then I would set the idle for maybe
900 rpm for pre-1981, so the idle rpm will not increase too much going down in
altitude. I personally prefer about 1050 when doing idle settings at sea level, or up
to a few thousand feet. Don't set nor use you Airhead for an rpm under 850; too
slow means poor chain & sprocket oiling AND overly-sensitive carb adjustments,
& a more abrupt off-idle transition.
Once you get a bit experienced at this, you will realize that the IDLE MIXTURES
are also changing, & you can make a SMALL change to them. Those adjusting
AT high altitudes might want to make the idle mixture screw a WEE bit 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 3/16 turn maximum difference
here! If you are a beginner at adjusting carburetors, I suggest you IGNORE the
two paragraphs in this NERDY section
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, adjusted 1/10th of a turn at
a time, until the engine sounds the smoothest, & the idle speed the highest.
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
gentle seating, thus you won't mistake the proper setting from starting too far
in to begin with (trying to move it even further in, & NOT finding the correct
Thus, 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 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. 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, PARTICULARLY when you are very 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).
You want to end up with the idle mixtures correct, 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 almost always
better to start with it too far outwards. Once done correctly, however, you need make
only the tiniest 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 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 'off-idle' transition position.
Because of this, I recommend 1300-1800 rpm (I suggest using 1500). You can lock
the throttle if you have the factory friction screw 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 moving the throttle suddenly 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 will have SOME such vacuum changes....
less is better & unless horrible...you 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 (shut the engine off). 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 balances, 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.
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 and short out the other
other cylinder. You will make the same as before 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.
Be 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. Both are done at the same 1300-1800 rpm (YOU pick the rpm you like,
but I recommend ~1500).
a) Short the plug circuit, 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 to clear the engine if it needs it.
b) Short the plug circuit, as before, left, then right, then left, listening to the engine,
this time allowing only ~1 second of shorting time. You might need a tiny cable
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 the correct rpm.
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 shorting
wire to allow it to be done with a single screwdriver in each hand. This method, on
SOME DUAL PLUGGED bikes, may reduce the idle rpm so far that the engine stops,
which you do not want.
Some have tried 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, & this also eliminates the problem of
carboned-up lower plugs. If you use this method, you either need two extra shorting
adapters, OR, you can use some worn-out spark plugs (correct thread depth!)...&
use zero gaps (never re-use those plugs). This works OK.
Hint: install 4 each spoke adapters, one at each plug. Make up jumper
wires (two are needed), maybe 6 inches long or so; an alligator clip on each wire
ends. For each cylinder, clip a wire onto a lower spoke, & the other end of the wire
to a nearby cylinder fin. Do the synchronization & balancing work by having only to
alternately short the top plugs adaptors. If that does not work for the particular engine,
short both plugs on one cylinder at a time to the fins, with one screwdriver.
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.
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 & 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:
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 is 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
carburetor articles for info/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 carb articles for the butterfly installation.
Note, AGAIN, that the factory, OR SOMEONE ELSE, may have installed the
enrichener parts backwards, AND note that some are MIS MARKED. See my other
carburetor articles for PHOTOS of the wrong, and correct, ways.
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
06/01/2014: Recheck article.
08/03/2014: minor changes for clarity, section separation
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.
Modest amount of clarifications.
12/17/2015: Finalize meta-codes. Clean up article and left justify for smaller screens
© Copyright, 2014, R. Fleischer
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