Motorcycle Repair: Motorcycle repair, air screw, engine vacuum

Question
if carburetor slow-speed circuit has an adjustment screw on the engine side of the throttle slide, what does the screw conrol

Answer
Valine, it depends on what carburetor you are asking about. If you had given a specific make/model/year I could tell you specifically.

However, the idle mixture screws do one of two functions....

If the end of the screw is blunt ===) then it is an air screw. Turning it inwards makes the mixture richer and leaner when you back it out. It controls the air balance of the idle mixture.

If the screw is like a needle on the tip ===--  then it is a fuel metering screw. Backing it out richens up the mixture and turning it in leans the idle mixture out. It controls the fuel balance of the idle mixture.

In either case, the screw controls the balance of air/fuel for the engine at idle speeds. It only functions at a small range of rpms, so if the ignition timing is out of spec or there is an air leak or something else that is causing a change in the engine vacuum signals to the carburetor and/or the idle speed is set too high, then the idle mixture screws will be ineffective. In modified engines, the idle/slow jets may need to be changed to allow for the screw to operate effectively within a small number of turns (usually 1 to 3 turns out).

Bill Silver