Removing & plugging the Pulse Air & Evaporative emissions systems,
In your BMW Airhead Motorcycle.
Also: How these systems operate.
© Copyright, 2013, R. Fleischer
The Pulse Air system:
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 emissions (smog). No pump is used,
the air drawn into the exhaust port helps burn up any residual gasoline mixture at that point by means of venturi
action at that port. Fast-moving exhaust
gases over a small hole located in the vastly larger exhaust port,
acting like a whistle, suck in clean air via two metal can 'valves', these are located in the air-cleaner area (underneath the air cleaner element).
Carburetor venturi vacuum is transferred via rubber hoses to those metal cans, and it is the carburetor vacuum that operates the cans valves. This transferred vacuum operates those valves upon throttle back-off, where vacuum greatly increases, allowing the exhaust venturi action to 'suck' clean air to the exhaust port. The arrangement is simple, works OK, and supposedly the metal can valves system (not all 1980 models had the valves) will prevent back-popping that might be otherwise heard in the mufflers. Back-popping on the over-run, and at idle, is often noticed however.
Why plug & remove the system:
The 1978 and later airheads began to have concerns by BMW for exhaust emissions. By 1978 only some changes to ignition timing and flywheel markings together with cam advancement of 6° crankshaft had been done. The 1980's and later airheads (for the USA) ran a bit lean to meet emissions laws and regulations and tend run hot, ...in particular, the R100 series... where it has been speculated that the Pulse-Air Clean Air System is partly responsible for the head warping that has been noticed. It has also been speculated-upon that the extra heat in the exhaust port is transferred to the exhaust valve and then to the early 1980's troublesome exhaust valve seats, possibly warping them as well, and otherwise contributing to valve problems. Thus it is felt that removal of the pulse air system might increase exhaust valve and exhaust valve seat life, and minimize head warping. This may well be so even after replacing the early 1980's faulty valve seats....which is quite commonly and nearly universally done, particularly on the R100 engines. Removal will also eliminate any tendency for back-snapping in the exhaust 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):
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 this system, but that in the past a private owner could, and the motorcycle probably would still pass smog inspections, due to how these tests are done. Do this conversion at your own risk. NOTE that the PRESENT laws probably DO 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!
There are various methods of removing and plugging the system, totally, or in part. I will try to list all of them.
(1) Plugging the Pulse-Air part of the system:
This is a simple solution. It maintains the stock appearance which many enthusiasts like for their BMWs. Undo either end of the metal pipe on each cylinder, and insert the proper sized steel ball bearing at one fitting. 5/16" is around the right size. Another possibility is of simply bypassing the vacuum control to the valves in the lower air cleaner area.
(2) Removing/plugging at the
You can cut the pipe close to the fitting at the head, pinch the pipe (I suggest also folding it over) in a vise, and possibly even braze it for 100% sealing. In some respects this is a good way, as the steel fitting that screws into the head is often well seized into the head, and requires 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. ***NOTE!!!!!....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 aluminum threaded hole of the cylinder head. Some have been fitted loosely, and loosen with the pipe fitting, and the steel adapter falls out and is lost on the road....some remove very easily; but most freeze-up in place. The combustion carbon helps the freezing-up, besides the dissimilar metals: steel and aluminum.
(3) Full removal:
The very neatest method of removing the Pulse-Air system is a total removal of all the pulse air parts, and installing a plug into the head. If you get the head hot enough, the aluminum will expand much more than the steel adapter, and make things easier, and help avoid thread damage. Immediately after riding, with the head hot, is a good time to do it. A socket wrench is used on the steel adapter in removing it. Only a moderately high amount of pressure using a socket should be used. If it 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...or Liquid Wrench...or Kroil. If you apply too much force, you will twist the adapter out, injuring the aluminum threads. It takes a fair amount of force, actually, to reach that point, but many have found how much it takes. You can also remove the 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 [(1) above], or find a brass threaded cap to fit over the existing steel adaptor, or cut the tubing and pinch it closed, as mentioned previously in (2). 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 an American pipe is. 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, ETC.
Removing the rest of the system:
Removing the parts in the air cleaner housing is optional. Removal of the cans gets obstructions out of the way, simplifies and neatens things, and might even aid carburetion a bit. Remove the air cleaner top, and the air filter. Remove both metal valve cans, and their plumbing. Do NOT remove the breather hoses and fittings. The breather system is separate, and its components easily identified, these connect to a LARGE rubber hose going WELL FORWARD of the air cleaner.
Remove the 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 that will push over that T and seal it. This will work OK, and you then don't need the carburetor vacuum tap screws listed below. While this 'couples' the two carburetor venturi vacuum takeoffs together via that hose, this works very nicely, no bad effects. You do not want unfiltered outside air getting to underneath the air cleaner, after you are all done....so, be sure that you do nothing that allows that situation.
NOTE!!....If you WANT TO, you may remove the very flexible
small diameter rubber hoses to the carburetors
themselves; and, plug each carburetor vacuum port with the tiny screw listed
below. 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, if you leave the hoses connected and plug the plastic T
in the air cleaner area. 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.....etc.
Install the following components:
1. 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, BMW #07-11-9-919-117. Install these with lots of Loctite RED 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 oil drain plug 07-11-9-902-292 will work, but have not checked on this myself.
***Oak Okleshen, firstname.lastname@example.org, has the plugs in stainless steel, but they are not cheap.
NOTE!!.....if you install plugs mentioned above into the aluminum head, be sure to recheck tightening when the head is still HOT from riding....otherwise it MIGHT come loose. USUALLY the 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 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 as I noted, I am not sure if it is all that effective, as the port gets pretty hot...and Loctite loosens above 300 degrees or so; but wouldn't hurt. I prefer to install them with Muffler Mender type of cement.
2. Two each black rubber timing plugs, BMW #11-11-1-744-327 to plug the holes at
the air cleaner housing. I glue them in place with black RTV
rubber on the outside and copiously on the inside, and under the plug head. 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.
3. If you decide to remove the flexible hoses from the carburetors, you will need two each vacuum port screws for the carburetors, BMW #13-11-1-259-869. You do not need these screws if you leave the interconnection vacuum hoses intact, which works just fine (plug the unused T port facing rearward, as above described). These screws are hard to find in hardware stores. They are 3.5 x 0.6 mm, and about 5 or 6 mm long. Try to find steel ones.
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:
> 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.
Yes, it is 16 mm 'fine' thread (1.5 mm pitch). I have seen this happen more than once...to say the least. It USUALLY happens 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 glue.....and hardens to a rock. I used to recommend that folks only cautiously remove that steel fitting...and to try removing it with the head HOT first, and if no luck, try multiple soakings of some sort of 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 (not tapered) 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 typical ripped 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 my last one at a neighborhood 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: inserted 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 are minimal as reformed;.... and I tighten only modestly tight. I usually recommend that Loctite RED be used (I use muffler cement!!).
I 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, silver solder or braze it shut, and install it that way. Or find a cap fitting as mentioned. Granted that 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. Of course, you can use the fitting from the original pipe (cut the pipe to remove it) and weld or braze the hole that the pipe went through.
*****NOTE!!.....if you install the
plug mentioned in this article into the aluminum head, be sure to retighten it
when the head is still HOT from riding....otherwise it MIGHT come loose.
The Metric & MultiStandard Corporation carries just about everything in metric fittings. They even carry the 52 mm dies to reform the exhaust port threads (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
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.
Bottom line: I suggest you NOT remove the steel fitting in the head if moderate force does not allow its removal.
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 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!....because 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....and...>>>....well, read on....
The Evaporative Emissions System:
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 had two additional ports under the tank, two solenoid valves were added, a port into the crankcase was added (with a relief valve), and one port into the air cleaner area. This system is totally independent of the Pulse Air System described in the above section of this article. The purpose of these new items is to prevent fuel fumes from escaping into the atmosphere. The fuel cap and venting on the fuel tank was now sealed. As heat from the sun or atmospheric or engine heat causes the fuel tank fumes to be pressurized, the fumes are directed to the crankcase. There are two vents (or, call them diaphragm type valves) in the 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 damage. In addition, the fuel tank is designed with a flapper in such a way that the tank can not be overly filled...this reduces tank capacity, but allows for an adequate non-liquid area for compressed fumes, and will prevent spraying fuel if the cap is opened when the tank is a bit pressurized with fumes. Perhaps the allowed volume is larger than need be.
Below the tank and in the starter motor area are the 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. 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 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 float needle valve sealing.
have removed one or all of these parts. If you do
remove parts, BE SURE to cap the short vertical pipe leading
crankcase. Some remove the flapper valve in the fuel tank, there are two
types, keeping them intact prevents filling the tank to capacity. If someone has
already modified your system, be sure that vertical crankcase pipe vent is capped and the
cap in good condition. Modifications done are usually of the type that
returns the system to the old way, before this stuff was added. Be
sure not to fill the fuel tank without leaving 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. At least 1/2" of air space is needed,
otherwise the fuel can heat from the engine, sun, etc., and overflow.
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: : Remove troublesome language button, minor editing for clarity.
Copyright, 2013, R. Fleischer
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