Mukuni VM: Outside Parts and Functions
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Most of the external parts of a Mikuni Round slide (valve) carburetor are shown above. The starter enrichment valve, is more often called a "choke"- (though it really isn't). (Click the imbedded link for more information.)
These carburetors are differentiated based on their throat size. It can be measured on the side facing the engine. (This is a 30 millimeter carburetor.)
The fuel inlet "barb" is the tube seen here. The black cap covers a port which opens into the carburetor throat. It can be used when calibrating the carburetors. Normally it must be kept capped.
These carburetors don't have many external adjustments. There are two screws visible in this picture. The big one, the one with both a Phillips screw drive and finger grip, is the idle screw. Turning it in (clockwise) increases idle speed. Tinkering with this screw is not usually a problem. The only issues to be aware of are these:
- both carburetors should be adjusted about the same
- don't increase idle speed to more than 2000 RPMs to prevent possible unexpected clutch engagement. (Clutch engagement is usually 3500-4500 RPMs.)
- it is possible to have safety switch problems if the idle is raised too much. It is best to keep the idle speed at around 1500 RPMs.
The second screw, the smaller one with the slot, is the air screw. This screw is not one to be tinkered with. It controls air flow to the pilot jet which functions in the idle and low speed operation of the engine. It should be adjusted and then left alone. There is nothing to be gained by adjusting it. If it is originally adjusted properly, any future improvements are an indication that there is some other problem in the carburetor or the engine.
Never Tighten this screw! Refer to the specifications for your engine in order to set this screw. The screw is properly set when it is turned out a specified number of turns - usually between 1 and 1 1/2, but sometimes as low as 1/2 or as high as 2 turns. Refer to the engine specifications. The screw should first be turned in until it is lightly seated, then backed out the appropriate number of turns. Refer to a proper source for further tuning instructions.
The small brass "barb" directly above the air screw connects to a passageway which provides atmospheric pressure to the float bowl.
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