BMW Carburetors

BMW applications generally use a  two barrel " down draft " style carburetor originally manufactured by Solex.

This style of Solex carburetor is unique in that (unlike most carburetors) you do not adjust the throttle stop screw to set the idle speed. 

Instead, these Solex carburetors have a large brass screw which sets the air delivery at idle. This "air" adjustment  is used in conjunction with a traditional fuel mixture screw.