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The Pulse Air, Evaporative Emissions,
and Fuel Shutoff systems
Includes comments on fuel tank 'flap' valves, petcocks, & SHED caps, etc.
How these systems operate, how to modify them, etc.

Copyright, 2014, R. Fleischer

For best knowledge on these systems, read the following article (1B): 

BMW started to have concerns around 1978 for exhaust emissions standards being implemented around the world, this was particularly so in the USA, in the State of California.  By 1978 only some modest changes to ignition timing and flywheel markings together with 3 cam advancement (equivalent to 6 crankshaft) had been done.  The 3 advancement was done by re-working the camshaft sprocket keyway position.  The 1980's and later Airheads carburetors were jetted to be leaner; and that helped more to enable meeting emissions laws and regulations.  However the engines tend run hotter, particular, the R100 series, and more-so the faired models.

The Pulse Air system, description:
Beginning with the 1980 U.S.A. Airhead motorcycle models, BMW incorporated a system that "sucks" clean air drawn from the air cleaner area into the exhaust ports. The purpose of this nearly completely passive system is to reduce certain components of the exhaust emissions (smog). No air pump is used, the oxygen in the air drawn into the exhaust port via venturi action helps burn up any residual gasoline at that point.   This 'Venturi' is created by fast-moving exhaust gases that go over a small hole located in the vastly larger exhaust port, the hole acts like a whistle or old fashioned garden-type sprayer, creating a mild vacuum that sucks-in clean air via two metal can 'valves' which are, themselves, specially inter-connected.   The metal can valves are located in the air-cleaner area underneath the air cleaner element.  Since any burning of residual fuel in the exhaust port is not being burnt in the combustion chamber, the exhaust port area can have a considerable heat increase, some of which may affect the area of the entire exhaust valve, valve guide, valve least that is the theory behind those wishing to eliminate the Pulse-Air system; which often causes mild backfiring (back-snapping) during throttle closures.   The valves need something to turn them on and off.  This is done by another venturi vacuum generator, which is a rather small hole in the carburetor, at the bottom of the throat, very close to where the carburetor butterfly closes.   This system is only used on the 1980 and later models, ALL of which use Bing CV carburetors. ALL these models having the Pulse-Air system have rectangular airboxes...NOT the earlier clamshell airbox.

Carburetor venturi vacuum is transferred via very flexible small diameter rubber hoses to those metal can valves.  It is the carburetor vacuum that operates these valves.  This transferred vacuum operates those valves upon throttle back-off, at which time carburetor venturi vacuum greatly increases, and thus the valves are operated. 

In case you are confused:  When the exhaust gases pass over the small hole in the cylinder head exhaust port area, that creates a sucking effect, sucking in air supplied by the can-valves, if they are in the ON position; and for them to be in the ON position, the carburetors must be supplying vacuum to those can-valves.  That vacuum moves a diaphragm inside the valves, opening the valves to let in air, which contains oxygen, which then travels to the exhaust port which is trying to suck such air to it.

The arrangement is simple, works OK for reducing smog-causing emissions, and supposedly the metal can valves system (not all 1980 models had the valves, and European bikes did not come with it in the beginning) will prevent back-popping that might be otherwise heard in the mufflers. Back-popping on the over-run, and at idle, can be noticed however, sometimes worse with some motorcycles than others.

Why plug & remove the system (besides wanting to eliminate the backpopping)?
It has been speculated that the Pulse-Air Clean Air System is partly responsible for the head warping that has been noticed, especially on R100 engined-models.  It is believed that the extra heat in the exhaust port is transferred to the exhaust valve and then to the early 1980's (through 1984) troublesome exhaust valve seats, possibly warping them as well, and otherwise contributing to valve problems.  Thus, quite a few folks, including professional mechanics and guru's, think that removal/plugging of the pulse air system might increase exhaust valve and exhaust valve seat life (even on 1985+ models with the improved valve seats), and also minimize head warping.   These things may be so even after replacing the 1980 to 1984 faulty valve seats with the 1985 and later seats which do not have problems. 

Removal/plugging of the Pulse Air system is quite commonly done.   This will also usually eliminate any tendency for back-snapping in the exhaust during trailing throttle (if the PulseAir System was causing it).

Will I have to change jetting or settings of the carburetors or ignition?... NO

Is this a difficult job?... NO

Will it stop the back-snapping in the exhaust if due to the PulseAir system?...  YES

It is my understanding, and I am NOT an environmental lawyer (nor any other kind of legal expert), that a Dealership COULD  NOT legally remove the Pulse Air system, but that in the past a private owner could, and the motorcycle probably would still pass smog inspections, due to how those tests are done. Do this conversion at your own risk.  NOTE that PRESENT laws may...or may not... prevent you from doing this removal for road use, as opposed to closed course racing.  I am neither recommending you do this conversion/removal, or not, this is educational material.

Pulse-Air system removal:

There are various methods of removing & plugging the Pulse-Air system, totally, or in part:

(1) This method is very simple & maintains the stock appearance. Unfasten either end of the metal pipe on each cylinder, & insert the proper sized steel ball bearing at one fitting.  5/16" is around the right size.   You can get steel ball bearings from your local hardware store, or a hobby crafts store, or even maybe some local Wrench has some bad bearings to remove a ball from.  You can also just tightly fit some sort of small plug into the pipe.

(2) You can cut the pipe close to the fitting at the head, pinch the pipe (I suggest also folding it over and pinching it again) in a vise, and possibly even braze it for 100% sealing (NOT necessary with a double pinch).  In some respects this is a good method, as the steel fitting adapter that screws into the head is often well seized into the head, and may require a lot of force to remove, and using a lot of force may strip threads, necessitating purchase of a 16 mm x 1.5 mm bottoming or plug tap. The adapter that screws into the cylinder head is steel, and tends to gall/weld (being helped along by combustion byproducts including burnt oil/carbon) to the threaded hole of the cylinder head.    Some remove easily.  Most freeze-up in place.  The combustion carbon 'helps' the freezing-up, besides the dissimilar metals problem.    You do not want to force that adapter out of the head, if you do, you may strip the threads, which is, actually, usually not a disaster.  Read the rest of this entire article, there is information on getting that adapter out, etc., in numerous places herein.

(3) Full removal of all the parts:

This is a total removal of all the Pulse Air parts, and you then install a plug into each head. If you get the aluminum head hot enough, it will expand much more than the steel adapter, and make things easier, and help avoid thread damage.  Immediately after riding, with the head very hot, is a good time to do this.   A socket (NOT a box end or open end wrench) with handle is used on the steel adapter in removing it; you may get a wee bit of extra 'help' by putting the socket into your freezer, before the ride.  Only a moderately high amount of pressure using a socket should be used. Some have tried dry ice against the adapter, then quickly using the socket.  If the adapter does not remove right away and relatively easily, then try again, after perhaps a day or week of repeated soakings (engine cool) with a mixture of acetone and automatic transmission fluid (50-50).  That is the best mixture; but you can try Liquid Wrench...or Kroil or other favorite penetrant.

If you apply too much force, you could damage the threads in the aluminum.   It takes a fair amount of force to reach that point, but some have found how much force it takes.   You can also remove these adapters, this is usually done with the heads off the bike, by heating the steel to red-hot, then splashing water on it.  You can try with the head on.   If your steel adapter seems quite well frozen, it may be wiser to use the ball bearing plugging method, or find a brass threaded cap to fit onto the existing steel adaptor, or cut the tubing and pinch it closed, as mentioned previously.   Some have spent the time to find a properly threaded (usually brass) cap, but note that for 3/8" standard pipe caps, the threads are NOT the same!  The proper cap is 16 mm x 1.5 mm thread pitch, and the threads are NOT tapered like many standard American pipe fittings are. Metric specialty stores may carry these caps.  I have used the brass ones.

(4) There are other ways of plugging, such as brazing the nut that holds the pipe to the steel fitting, without using the pipe stub.
Another possibility is simply bypassing the vacuum control to the metal can valves in the lower air cleaner area. If the cans do not receive carburetor vacuum, they simply do nothing.

(5)  Removing more of the various parts:
Removing the parts in the air cleaner housing area is optional.  Removal of the vacuum valve cans gets obstructions out of the way, simplifies and neatens things, and might even aid carburetion a bit.  To do this:  remove the air cleaner top, and remove the air filter. Remove both metal valve cans, and their plumbing. Do NOT remove the breather hoses and fittings. The breather system is separate, IT IS DESIRABLE, and its components easily identified, these connect to a LARGE rubber hose going WELL FORWARD of the air cleaner.       While you are in the air filter housing area, look at the inside bottom, center, rear area.   There is a 'funny-looking' rubber part, that is a tiny dual-flapper-valve, a sort-of-drain device.  These deteriorate over time, and can then allow dirty air into the engine!   Either replace it, 13-72-1-337-162, or plug the hole it was in.

Remove the pipe fittings on the air cleaner lower housing.  Remove the flexible rubber hoses that connect from the SMALL plastic T adapter to the vacuum valve cans.   PLUG the rearward facing port of that small plastic T.  All automotive parts stores carry small black plastic covers (vacuum pipe caps) that will push over that T and seal it.  This will work OK, and you then don't need the carburetor vacuum tap screws and washers listed below.  While this 'couples' the two carburetor venturi vacuum takeoffs together via that hose, this works very nicely, with no bad effects.  You do not want unfiltered outside air getting to underneath the air cleaner, after you are all, be sure that you do nothing that allows that; that is, all air going to the carburetor intakes must come from and through the air filter, without outside air entering any holes, etc.

NOTE!!....If you WANT TO, you may remove these very flexible small diameter rubber hoses that went to the carburetors; and, plug each carburetor vacuum port with the tiny screw and washer listed a bit further, below.  AGAIN:  Be SURE that you leave no way for UNfiltered outside air to get under the air cleaner element.   You really don't have to plug those carburetor holes with the screws and washers, if you leave the hoses connected and plug the plastic T in the air cleaner area as in the prior paragraph.   If you don't want to mess with easy to lose small screws (and washers, if you add them, which are not really necessary) when synchronizing the carburetors (if you use the vacuum methods), then leave the hoses intact at the carbs....pull them off when attaching vacuum synchronization devices..

What you will or might install?:

If you removed the steel head adapters, then you need two plugs for the heads, these are 16 mm size straight thread (1.5 mm) plugs, similar to the drain plugs used for the R11 series, K bike series, and many European cars.  Install these, 07-11-9-902-292, with lots of Loctite RED (any of the very strong higher temperature red Loctite's) and a 07-11-9-963-252 crush washer (16 x 20 mm) on each plug.  YOU DO NOT HAVE TO USE THESE CRUSH WASHERS, and may well not want to if you had to retap the threads and there are limited number of threads left.  It has been reported to me that the newer numbr oil drain plug 07-11-9-902-292 will work, but have not checked on this myself. 

***Oak Okleshen,, may still have plugs in stainless steel; they are not cheap.

NOTE!!.....if you install plugs into the aluminum head, be sure to recheck tightening when the head is still HOT from riding....otherwise it MIGHT eventually come loose.  USUALLY red Loctite sets-up and you won't be able to tighten more.   Check it after a ride or two, and then yearly.   I install plugs with the head moderately hot right from the get-go.   Torque is about 20-25 footpounds.  DO NOT USE ANTISEIZE ON THIS PLUG.  While Red Loctite can be used, I am not sure if it is all that effective, as the port gets pretty hot...& most types of Loctite loosens above 300 degrees or so (#263 may be better); but wouldn't hurt.  I prefer to install them with Muffler Mender type of cement!

Two each black rubber timing plugs, BMW #11-11-1-744-327 to plug the holes at the air cleaner housing.  Clean the holes on outside and inside with a good solvent so the RTV will stick well.  Put some black RTV on the sides of the smaller diameter end and then push into the hole from the outside.  If you want to, and I do this, add more RTV on the inside.  I clean off the excess RTV on the outside with acetone before it hardens.   I find that RTV works over the long term, Crazy Glue does not.

***NOTE: as an option, one could use the Euro airbox, 13-72-1-337-250 which does not have the smog parts holes....this is a $$$ part.

If you decide to remove the flexible hoses from the carburetors, you will need either two common vacuum port closed-end rubber or plastic covers (any auto-parts store)....or; two each vacuum port screws, BMW #13-11-1-259-869.  These short little round stub pipes at the carburetors are internally threaded.    You do not need these screws if you use the covers, or leave the interconnection vacuum hoses intact, ALL are OK methods, but do be sure to plug the unused T port facing rearward, as previously described. These carburetor vacuum port screws are often hard to find in hardware stores in the USA.   They are 3.5 x 0.6 mm, and about 5 or 6 mm long.   Try to find steel ones, or just use the BMW ones, #13-11-1-259-869.

NOTE: if you are anal enough, you can also obtain two flat washers for these carburetor port screws, BMW #13-11-1-259-870, but they are not really necessary.


NOTE!    This is an edited version of an inquiry...and my reply to the Airheads LIST (edited for this article):

> Hedz,  The threads in one of my heads for the emission plug is bad.  The first half of the threads are ripped out. They are 16mm with 1.50 pitch. Does Helicoil make a kit? Timesert does not. My PC keeps freezing up when I try a search.
> I don't want to weld them up....yet.

my reply:
Yes, it is 16 mm with 1.5 mm pitch.  I have seen this happen, USUALLY because the fitting that goes into the head is a type of steel, and also combustion carbon gets into the threads and acts like a wonderful very hardened glue.      I used to recommend that folks only cautiously remove that steel fitting...and try removing it with the head HOT first, and if no luck, try multiple soakings of some sort of penetrant, like 50-50 acetone and automatic transmission fluid, or Liquid Wrench or similar oil to soften the carbon...but often that does not work, even over a week's time.    Folks have stripped those threads.   I have repaired a number of them...and have never had to use a Helicoil type of insert.   BUT... below I will give the information on where to purchase such inserts/kits.      NOTE that 16 mm x 1.5  mm straight thread (not tapered thread) metric brass caps are available (not easy to find though) that will cap off the steel fitting which need not then be removed.  This is NOT as 'neat' as removing the steel fitting from the head, but is totally safe.     For fixing typically stripped threads,  I have a 16 mm x 1.5 mm 'plug' tap.  Actually mine is a cross between a plug and bottoming tap.   They are not difficult to find.  I purchased mine at a hardware store.    PLUG/BOTTOMING means that the working end of the tap is not very tapered....just a small amount.  That enables the tap (CAUTION: insert SQUARELY and with some goodly force), to grip onto any remaining threads or partial threads, and re-cut and re-form them.      A regular 'starting' tap has so much taper that you can't use it, as the port's hole depth is not enough. Once the threads are reformed, I install the new drain plug, see well above for BMW part numbers;...usually withOUT a gasket, as usually the threads depth are minimal as reformed;.... and I tighten only modestly tight.  I usually recommend that Loctite RED be used (but I personally prefer and use muffler cement).   
Because of ham-fisted folks that do not know how much force they can use before stripping threads, I now mostly recommend that folks leave the steel adapter in the head, and cut off the end of the attaching pipe to a VERY short piece, flatten its end, double it over, flatten again...or, just silver solder or braze it shut.  Or find a cap fitting.  The drain plug installation is far neater-looking.  You can ALSO use the old fitting that was on the PIPE, remove the pipe (cut it), and use a ball bearing in the fitting in place of the pipe.  You can also use that fitting and weld or braze up the hole in the fitting that the pipe went through. 

*****NOTE!!.....if you install the plug mentioned in this article into the aluminum head, be sure to re-tighten it when the head is HOT from riding....otherwise it MIGHT come loose.

The Metric & MultiStandard Corporation carries just about everything in metric fittings.  They even carry 52 mm dies to reform the exhaust port threads where the finned nut screws on...(VERY pricey, too!).  They have an 8140 series of inserts (yes, like Helicoils)....and also the various components such as the special taps, tools, etc.  Also kits...yes, in 16 mm x 1.5 mm.      $$.  They have warehouses and offices all over, but here is the main number:  1-888-966-MMCC.   While a great source for things, you can find the bottoming tap at many hardware stores.

I suggest you look at your threads carefully....see if you can do the repair with a 16 mm bottoming tap (mind the caution about squarely). Maybe $12 total for a bottoming tap....versus a LOT more for doing an insert....unless you find someone with the tools and inserts....even then, I hardly see any really good reason to do an insert, unless the threads are damaged way beyond using a tap (I have yet to see them THAT bad).

Bottom line:  I suggest you NOT try overly hard to remove the steel fitting in the head... if moderate force does not allow its removal, head fairly hot.

Do NOT modify the engine breather system!....that is the larger black hose that goes far forward into the starter area system!...and has hoses leading to funny-looking plastic finned adapters, located in the carburetor outlets of the air box. 

While working in the air cleaner area, you may want to remove the starter motor cavity cover (disconnect battery negative wires first!!!)....then tighten the starter motor electrical nuts.  Two things to especially look for: one is that the large wire to the solenoid is not in danger of shorting to the cover or case; and secondly see how the starter cover fits.  Sometimes the cover fits very tightly against the air cleaner housing....some judicious hand-filing will make things easier.   Heck, why not remove the fuel tank and service other things at this time too!...maybe your electronic ignition module needs fresh heat sink grease, or it is time to clean and coat the electrical connections, inspect coils and wires; whatever; ETC.   WHILE THE STARTER MOTOR CAVITY COVER IS OFF, YOU HAVE ACCESS TO THE TWO SOLENOIDS USED ON LATER MODELS FOR CRANKCASE FUMES AND FUEL SHUTOFF.  MORE ON THAT LATER IN THIS ARTICLE.


Here is a link to Scot's article on what he did, with some good close-up photos.  You may want to do something like Scot did, or, just get ideas:

Snowbum does it a bit differently.

The Evaporative Emissions System:
This system is separate from both Pulse Air system & breather systems.  Some may want to remove all, or partially.

Generally beginning with 1985 models sold in California, and 49-State models sold in 1986 (not the R65), a new system was incorporated.  The gas tank now had an additional port under the tank (left side, two downwards short pipes), two electrically operated solenoid valves were added, a port (upwards facing short pipe) from the crankcase was added (with a relief valve), and one port into the air cleaner area.  Components and hose holes, etc., were installed into the starter motor top cover.  These systems are totally independent of the Pulse Air System and main breather system partially described as to its hose in the above section of this long article.  The purpose of these new items are two-fold:  (a) to prevent fuel fumes from escaping into the atmosphere; and, (b) to turn off the fuel via a solenoid (the petcocks remained!).   The fuel cap and venting on the fuel tank were now sealed unless tank pressure or vacuum got high.  As heat from the sun or atmosphere or engine heat causes the fuel tank fumes to be pressurized, the fumes were directed to the crankcase through an electric valve that was in the open position with the electrics turned off. 

There are two diaphragm type valves in the fuel tank cap, one opens, allowing in outside air, at a vacuum of about 0.1 Bar (vacuum of 1.5 psi), this is to allow for the fuel being used during engine use.  If the pressure in the tank, perhaps from being in the sun, reaches 0.3-0.4 Bar (4.4 to 5.8 psi), the second valve opens as a safety, to prevent tank or other damage.  

The fuel tank is designed with a metal flapper in such a way that the tank can not be overly filled...this reduces tank capacity, but allows for a more than adequate non-liquid area for compressed fumes, and thus the tank filler area flap MIGHT prevent spraying fuel if the cap is opened when the tank is pressurized with fumes. The flapper-mandated "no-fuel filled" volume is rather considerably larger than need be.  Thus, in the stock tank, you can NOT obtain the full amount of gasoline into the fuel tank.  It is best to NEVER 100% fill a tank anyway, as it can pressurize under some circumstances, which can create various problems.  The flapper is installed immediately below the bottom of the round entrance to the fuel tank, and this bottom area has a SMALLER diameter, so that old-time leaded-fuel large size fillers at the gas station will NOT fit deeply into the new tank.  There is a photo and text, later in this article, about ways to get around various problems.  Some remove the flapper valve in the fuel tank, there are two types, keeping them intact prevents filling the tank to capacity.  Actually, you should always leave a little bit of air room in the tank, filling into the fuel filler neck area maybe half way or bit more is acceptable, but hard to do if the flapper is intact. 

Below the tank and in the starter motor cover area are these two solenoid valves which are electrically operated.  One of these valves, called the air vent valve, either passes fumes to the carburetor via the air cleaner area (ignition ON), or to the crankcase (ignition OFF).  This valve is connected to a port on the fuel tank, underside, that was not on earlier fuel tanks.   The connection to the crankcase is interrupted by a pressure relief valve (not electric), opening at about 0.15 Bar (about 2 psi), and this also prevents reverse flow of fumes from the engine operation.  Note that this particular relief valve has an arrow on it, pointing towards the engine, that is, DOWNwards.

The other solenoid is a fuel shut-off device, and it is located in the fuel supply physically below the manually operated fuel petcock valves. Yes, that is in addition to the petcocks.   Idea (well, ONE of them) is to prevent carburetor overflow or dripping from insufficient carburetor float needle valve sealing, if the petcock(s) are left on.

***These horizontal outlet late style petcocks are HANDED, that is, there is a left hand side and a right hand side.  This information is hardly noted anyplace but here by me, >>>as is why TWO right sides are used on some bikes.  When installing to the fuel tank, with the fuel solenoid removed, if you install the left petcock so the OUTLET is pointing REARWARDS, then the curve of the fuel hose will be much nicer, shorter, and the hose might last longer, and less hassles with any aftermarket fuel filter. That means the left petcock must be of the slightly older LEFT TYPE.

xplained slightly differently:
In mid-1980, and later, BMW incorporated the tank fumes solenoid valve and a fuel flow solenoid valve as described.  These are located on the cover that fits over the starter motor, and there are holes in that cover on these models for hoses, etc.   Because of the arrangement of hoses, etc., BMW elected to use TWO EACH "Right Hand Side" petcocks.   If you remove some or all of these solenoid parts, you MAY want to purchase a LEFT hand side petcock, which COMES with the filter screen.
It makes for a neater arrangement if you've removed the electric solenoids, etc.

Many have removed one or some or all of these parts.   If you do remove parts, BE SURE to cap the short vertical pipe leading into the crankcase; you can use a common automotive type of closed end vacuum cap; just slide/push it over the pipe. Many just use a short piece of rubber hose, insert on the pipe, bend the upper part of the hose over, along side the hose, and put a strong zip tie on it.      Be sure however you do this, that it is a long lasting affair.  You can also crimp the pipe instead.  If someone has already modified your system, be sure that the short vertical crankcase pipe is capped and in good condition, that is, it must NOT be just open. 

Modifications done are usually of the type that fully returns the system to the old way, before all these items were added by BMW.  Be sure, as always in the past, not to fill the fuel tank without leaving at least a bit of air space below the top surface of the tank.  This means you can fill into the cap screw area, but NOT to the actual tank metal top.  ~ 1/2" of air space is needed, otherwise the fuel can heat and expand from the engine, sun, etc., and overflow; and if it does so, it will come out the overflow pipe usually.  

The existing newer-style fuel cap can be used as is or you can modify it...OR NOT.  There have been problems with them, if NOT modified.     If you wish to defeat the SHED function (as it is called, and the cap so marked at its bottom), and convert for a vent, you do that by drilling a hole into the bottom of the metal bottom portion of the SHED (so marked) cap.  Drill an ~1 mm hole, centered, through the first layer of metal, and just through the second layer. Do it with a hand drill, as it is near impossible to feel for the correct depth on a drill press.

If you have the more common flap, it is built into a welded short section of the tank filler pipe (where the cap screws into).  You can use a screwdriver, pushing on the flap at perhaps 8:00 or 4:00, and pry the flap sideways some, and then at the 6:00 position you will see a small rectangular opening in the flap rear part, stick the screwdriver into it, and pry the flapper from its mount.  It will fall into the tank unless you have bent the flapper so it can be pried out (which is difficult).  The size of the large hole just above where the flapper was, see the photo just below, is sized to fit UNleaded fuel dispenser hose nozzles.  There is a section in this article more fully describing the flapper, a few paragraphs below the photo. YOU CAN use large nozzles, but can't insert them fully.  So, read the text that follows, AND, the text underneath the photo.

NOT mentioned anyplace but HERE, the following method I figured out will greatly help filling the tank, no matter to what level.  Drill some holes around the periphery, at the bottom area.  Here is a photo of a crude way I did it. YOU can drill a bit neater.  This will allow filling the tank much higher, due to the ability of the displaced air to return in much higher volume during re-fueling.  You can leave the steel filings and flapper, if you wish, in the tank, cleaning them out eventually, perhaps, at your yearly tank cleaning.
READ the text below this photo!!!

Here, the flapper has been removed,
and 7 holes drilled. 6 to 8 is fine.
Here, the 8th hole, as noted as drain 
at about 7:30, is the stock water drain
hole, it leads to a pipe stub under the tank.
That water drain hole/pipe was STANDARD
on all screw-top fuel tanks, even before
the above version of the filler neck.
The second pipe stub underneath the tank
is the actual tank vent, it is located near the
INSIDE TOP AREA.   As noted, for these
tanks, a SHED fuel cap is used, and you
may want to modify that cap as mentioned.
NOTE:  A FEW folks, maybe for use in a
FEW countries, may need to more easily
accommodate the old-style large filler tube
handle at the gas station pumps; OR,
simply want to cut out the bottom of
that filler area; which allows very fast
refilling at any rate.  For THEM, cutting out
the entire bottom area, where I show the
7 holes, above, can be done, either that,
or fill more slowly. To cut the bottom out,
use an approximate 1-1/2" round tubular


As noted, one electrical valve acts in concert with the fuel petcocks. You can remove one or both of the valves, as noted, but minor modification of the fuel connections are needed.  I regard the fuel solenoid to be a safety improvement, but I usually remove it; after all, you are SUPPOSED to turn OFF the petcocks when you park the bike!

You remove the electrical harness, and connect the remaining two plugs together, if removing the fuel valve.
That is a Adobe Flash video, so you may have to install Flash, or, have to do a right click to allow Flash to function, or?  Depends on your computer and your browser (CHROME usually plays it fine).    The video was made to show how "a problem" was fixed, but in the instance of just a removal of the fuel solenoid assembly, etc., you do what the video shows.

A link to Scot's article on what he did, with some good close-up photos.  You may want to do something like Scot did, or, just get ideas:

Snowbum does it a bit differently.

The tank CAP is special on these late models. 
So is the 'added' tank port, underneath.

1.  The cap may look like the ones on earlier models, but it contains TWO valves.  The cap itself is not openly vented to the to the atmosphere.  It is possible to do so, but you need to drill the cap and then have to deal with, somewhat, fumes.   A SHED cap says SHED on the bottom surface of the cap's metal end.

2.  One of the two cap valves opens at a vacuum of 1.5 psi in the tank to admit air to replenish the fuel you are using & allow fuel to flow to the carburetors.  The other valve opens if the pressure inside the tank exceeds something over 4.4 to 5.8 psi.   That is a safety feature for the tank construction, ETC.  Whether or not these valves in the cap are OK...or not...means little to nothing with the rest of these modifications AS DESCRIBED, assuming the cap is working properly.

3.   The TANK for these later models is also different. There is still the short metal pipe leading downwards, under the tank.  This is the overflow pipe, a short stub on the left forward under side on this type of tank, that vents the short cavity just at the top overflow area of the can see that area, and its downwards-going hole, when you remove the cap.  That hole is ~3/16" in diameter, and is at 7:30 on some tanks & 9:00 on others, approximately, as you sit on the bike.  A notation in red in my photo, well above, calls it the DRAIN.  The exit of that hole/pipe is that short stub under the tank.  You can leave it alone, or, run a rubber fuel hose from it to rear of the battery, downwards.  It is how water that gets under the cap, fuel splashing while filling, etc., is sent to the ground.  On early tanks, that short stub outlet is more to the right underside of the tank.

3.  The other short stub tube at the front left of the tank is the FUMES and FUEL overflow. IT IS NOT PRESENT IN EARLY TANKS.  In the stock version of later tanks, this tube connects via a hose to an electric solenoid valve.  That valve vents fumes to the crankcase via a plastic one-way valve. Remove it all, plug the tube coming up from the crankcase, and add a rubber hose, again to the rear, behind the battery and downwards.  See earlier part of this article on the air vent valve, in the section marked The Evaporative Emissions System.  You do not have to use rubber hoses, you can leave the stub pipes alone.  If you wish, you can use just ONE rubber hose to the rear, by using a T fitting....or, something like Scot put in his article:   I have not yet found any commercial T fitting that is simple and easy to use and fits.

It is theoretically possible to have the fuel catch on fire, ETC., while drilling or cutting into the bottom of the filler area.   While you should wear safety goggles, you may want to know how to best protect yourself from a gasoline fire/etc., without going to extremes, such as flooding an empty tank and dried-out tank with CO2, or other method.   The method shown below is pretty much standard on how to use an electric motor with a metal cutting disc, or with a drill bit, or, tubular saw, or, whatever.  I am careful, but have not done the following procedure, but YOU MIGHT WANT TO:

This is what I suggest, if you want to cut and remove the neck below the cap threads.  This is an example, and you could adapt 1-3 to what you intend to do, if it might create sparks.

1.  Drain out all the gasoline you can.  THEN REMOVE the petcocks.

2.  Remove the flapper, or leave it for now.

3.  Flood the tank with soapy water, several times, so no serious gasoline vapors are present.  You can now drill, etc.  But, if you are going to remove the neck below the cap threads, see 4. onwards:

4.  Using an electric drill motor (OR?), and a metal cutting disc, make a circular horizontal cut, through one wall of the filler, at a point BELOW where the cap threads are.  There is a limit to the diameter of the cutting disc you can use, which is OK.   While you could cut the tube all the way around, you will then be faced with grabbing the falling tube piece, and then squishing it to allow its removal from the tank.   The following method MIGHT be easier for you:

5.  With a cut through the wall, reasonably wide, you can use a screwdriver to pry the metal towards the center.  Be cautious!    You do NOT want to distort the tube where the cap screws into.  You might want to use something round to pry against, to avoid bending the tube where the cap threads-to.

6.  Continue the process, and remove the cut piece or pieces.

7.  Clean up the edges with a round file.

8.  Re-clean the tank.

9.  Re-install the petcocks (clean the screens if they happen to need it, etc.).  If your petcocks have the EARLY version of the tall screen that sits on top of the petcock, and that screen does NOT have a bonded washer, get the type with a bonded washer, and eliminate the original separate cambric washer.

12/14/2004:  incorporate all previous updates, and then revise slightly to be absolutely sure everything is
                    covered properly, in depth, and that there should be no confusion.
02/17/2005:  update, nothing but clarifications.
08/16/2005:  clarify the screw threads
09/24/2007:  minor typos fixed
04/21/2009:  Confirmed 5/16" ball size, fix minor typos and emphasis
04/30/2010:  Updated, mostly just clarified and simplified things
07/20/2010:  Add 07-11-9-902-292
04/14/2011:  Recheck for accuracy, minor cleanup/styling only
04/16/2011:  correct wrong 16 mm gasket number.
12/30/2011:  add that the plugs use a 1.5 pitch and add that Oak has the plugs in stainless steel.
03/24/2012:  check article over, no changes but to add a figure to how much air space is needed in the
                    tank (at end of article) & copyright year.
07/24/2012:  Correct the figures and how described in the Evaporative Emissions section.
10/13/2012:  Add QR code; add language button; update Google Ad-Sense code
2013, 3X:  :  Remove troublesome language button, minor editing for clarity; describe installing the black
                   rubber plugs better.
12/13/2013:  Update article to improve clarity.
03/29/2015:  Add note and URL to Scot's article, two places.
05/23/2015:  Add more info on the flapper and evap mods; and more info and a photo on 05/24/2015;
                    revise slightly for clarity on 07/21/2015.
10/19/2015:  update for changes in the website links, as some things have been moved about.  Added
                    some easier-to-understand additional explanations. Re-arrange article for clarity purposes.
03/15/2016:   Update meta-codes.  Clean up article.

Copyright, 2014, R. Fleischer

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Last edit of THIS page: Tuesday, April 26, 2016