Pulse-Air (Clean Air) System Description:

Beginning with the 1980 U.S.A. models, BMW incorporated a modification that injected clean air drawn from the aircleaner area into the exhaust ports. Only the rectangular air-cleaner motorcycles have this system. The purpose of this essentially passive system is to reduce emissions (smog). No pump is used. Air is drawn into the exhaust port (to help burn up any residual gasoline mixture at that point), by means of venturi action …of fast moving exhaust gases over a small hole strategically located in the vastly larger exhaust port.  Yes, this is like a whistle.

How is the process controlled?

Carburetor venturi vacuum (using the same carburetor port where vacuum gauge(s) are sometimes connected to synchronize the carburetors) is transferred, via a flexible rubber hose, to metal cans located inside the lower air cleaner box (underneath the filter).  Each can contains a diaphragm valve. The transferred vacuum operates that valve, allowing the exhaust venturi action to ‘suck’ clean air to the exhaust port when the appropriate vacuum is present (typically occurring when the throttle is rotated off or lowered). The arrangement is simple and passive except for the moving diaphragm. The purpose of the valve(s) is to prevent

 

 

popping noises in the exhaust…and they are not always perfect at reducing such noises. Not all 1980 models had these valves.

Why plug or remove the system?

The 1980’s and later engines run lean to meet emissions Laws and Regulations. This makes them run hotter. The R100 series especially tends to run hot, and it has been discussed that the Pulse Air System is partly responsible for the head warping that has been noticed occasionally. It has additionally been discussed that the extra heat in the exhaust port is transferred to the exhaust valve and to the early 1980’s already troublesome exhaust valve seats. There is a more in-depth article on the Author’s website: https://bmwmotorcycletech.info/valves.htm

 

There is good reason to believe the conclusions of these various discussions; and also to believe that even with updated valve seats, plugging the Pulse Air System is good for the engine…and, usually will eliminate most of the slight back-popping noises in the exhaust….which noises are probably not good for the exhaust system, especially the mufflers.

Legalities:

It is likely illegal to modify this system, at least by a dealership. It is my understanding, and I am NOT an environmental lawyer, that a private owner COULD remove this system. The present situation on legality for the private owner is unknown to ME. It appears likely that the motorcycle will continue to pass standard smog inspections for emissions content.

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

Plugging the system:
This is the simplest solution. It maintains the stock appearance. Undo either end of the metal pipes to each cylinder, and insert the proper sized steel ball bearing at the fitting, and reconnect. Thus, you need TWO ball bearings, one for each side of the motorcycle.

Removing the system:
Temporarily remove the aircleaner top cover and then 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 are easily identified, they connect to a larger rubber hose going FORWARD of the aircleaner. Remove the pipes/hoses going to the cylinder heads and remove the associated fittings on the aircleaner housing. You may or may not want to remove the flexible rubber hoses that connect to the vacuum ports on the carburetors…read this entire article before deciding on those.

NOTE! There is a European airbox available, for those who like things even neater, and don’t mind the cost, this box does not have holes to plug. BMW 13-72-1-337-250

Install the following at the airbox:

Two each black rubber timing plugs, BMW #11-11-1-744-327.   I like to ‘glue’ them in place by cleaning the airbox holes/area, where the pipes fitting had been, with a strong solvent, and then using black RTV on the rubber plug as fitted, and on the inside. Very little RTV is needed to hold them in place forever.

You now have decisions to make on how to plug the heads and deal with vacuum ports.

Methods for dealing with the heads….and some variations:

The steel adapters that are in the head can be removed with a socket wrench, but sometimes are well-seized from the effect of steel into aluminum and carbon deposits in the threads. If they do NOT remove with moderate force, don’t apply big force!  Try soaking the threads in a solvent (the best is 50% mixture of acetone and automatic transmission oil) over a period of time, even days of time, hoping that the steel adapters will release (they may not)….or you can force them out of the head …which can ruin the threads and you will need a BOTTOMING or PLUG style metric tap to ‘chase’ the head port threads. The tap needed will be 16 x 1.5 mm.

IF you manage to remove these steel adapters, the replacement is:
Two each threaded plugs, these are 16 mm straight thread 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 them with crush washers, BMW 07-11-9-963-252.
NOTE: If you have ruined the threads, and are re-threading using a bottoming or plug tap, you probably will have a minimum of operational threads left…in which case simply do not use the crush washers.

If you decide to leave the steel adapters in the heads:
You can plug them in several ways. One method is to get a metric pipe cap, they are usually brass…but possibly hard to find. Some folks have used a common 3/8″ American pipe cap, although the threads are NOT the same. Another method is to cut off the end of the old original pipe, just long enough to stick out of the old original nut on that pipe, and insert the pipe end piece into the nut, and squeeze the pipe end and possibly braze the end of the pipe. Another method would be to not use the pipe at all, just the original nut, and weld the nut where the pipe once existed. That turns the pipe nut into a cap-nut.

The final decision is what to do about the carburetor vacuum ports. There are TWO good solutions:

Install 2 each vacuum port screws (these are an odd-ball thread) for the carburetors, BMW 13-11-1-259-869.  Optional are a flat washer for each carburetor port screw, BMW 13-11-1-259-870; they are not really necessary.  The original hose pieces can be removed, together with the plastic ‘T’ piece joining them (in the air cleaner area). Make sure that no outside dirty air can can get to under the aircleaner element.  How you do that, is up to you.

In place of all of the above, simply re-join the highly flexible rubber hose originally used at the vacuum ports, but you will need to plug the rear-facing tiny port on the small plastic T in the aircleaner…any common ‘vacuum port plug’…a closed end plastic sleeve….available at autoparts stores…will work fine here.  The advantage of this method is that you won’t ever loose screws (and/or washers); & don’t have to worry about dirty air getting under the aircleaner, etc.  Stories abound, probably unjustified, that this method also helps make for a smoother idle.

Further information will be found in the Author’s website: https://bmwmotorcycletech.info/pulseair.htm.  That article has more details and also INCLUDES information on the electric solenoids that are located on later models, mounted on the underside of the starter motor area cover….how they operate, how to remove one or both, ETC.

Snowbum #1843   HTTPS://BMWMOTORCYCLETECH.INFO/INDEX.HTML