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Removing or Plugging the Pulse Air Injection System (AKA the Pure Air System)

Robert Fleischer (Snowbum)  –
Version 1.0 11-2-2001

Description of the system:

Beginning with the 1980 U.S.A. models, BMW incorporated a system that inserts clean air drawn from the aircleaner area into the exhaust ports. The purpose of this passive system is to reduce emissions (smog). No pump is used, and the air is drawn into the exhaust port to help burn up any residual gasoline mixture at that point, by means of venturi action, that of the fast moving exhaust gases over a small hole located in the vastly larger exhaust port. Carburetor venturi vacuum is transferred via a rubber hose to a metal can containing a diaphragm reed type valve located in the lower aircleaner area. This transferred vacuum operates that valve, allowing the exhaust venturi action to ‘suck’ clean air to the exhaust port. One for each carburetor is used. The arrangement is simple, passive, and supposedly the valves system (not all l980 models had the valves) will prevent back-popping that might be otherwise heard in the mufflers. Back-popping is often noticed however.

Difficulty of doing this modification: Beginner OK.

Why plug or remove the system:

The 1980’s and later airheads run lean to meet emissions laws and regulations. This makes them run hot. In particular, the R100 series tends to run even hotter, and it has been speculated that the Clean Air System is partly responsible for the head warping that has been noticed. It has also been speculated that the extra heat in the exhaust port is transferred to the exhaust valve and the early model troublesome exhaust valve seats, possibly warping them as well.

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


It is my understanding, and I am NOT an environmental lawyer, that a Dealership can NOT legally remove this system, but a private owner CAN remove this system, and the motorcycle probably will still pass any smog inspection. The author does not guarantee that it is legal to perform this modification.

Plugging the system:

This is the simplest solution. It maintains the stock appearance. Undo either end of the metal pipe on each cylinder, and insert the proper sized steel ball bearing at the fitting.

Removing the system:

  • Remove the aircleaner top, and the airfilter.
  • 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, they connect to a rubber hose going FORWARD of the aircleaner, and have fittings and hoses that connect to the large diameter carburetor supplying ports of the aircleaner box.
  • Remove the fittings on the aircleaner, and the pipes, and the fittings at the cylinder heads.
  • Remove the flexible rubber hoses that connect to the vacuum ports on the carburetors.

Install the following components:

  • 2-each, plugs for the heads, 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
  • 2-each, crush washers for the above plugs, BMW #07-11-9-963-252
  • 2-each, black rubber timing plugs, BMW #11-11-1-744-327
  • 2-each, vacuum port screws for the carburetors, BMW #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.

Robert Fleischer (Snowbum)  –
Version 1.0 11-2-2001

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