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Pulse air injection system; evaporative and fuel solenoids systems, as on BMW Airhead Motorcycles

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 bikes 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.

How is it 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).  The 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 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

 

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