Mikuni carburettors have become the standard brand of carburettor for high-powered motorcycles. Delivering high power fuel and air flow, Mikuni carburettors can rival and even surpass some fuel injection systems with their efficiency and power output. However, if the idle on your carburettor is not adjusted properly for your motorcycle, you may run into problems such as hard starts, sputtering, and high revs at a standstill. Learn how to adjust the idle on your Mikuni carburettor and make your bike run smooth always.
Set your motorcycle on its kickstand on a flat level surface. Locate the carburettor located between the air box and the cylinder head of the engine. If your motorcycle has multiple carburettors (such as some older Honda and Kawasaki models) you will need to make identical adjustments to each as you perform this procedure.
Locate the small Phillips head screw sticking out of the carburettor which is attached to the throttle arm. it should be easily recognisable as it will be the only one with a small spring wrapped around it. This is the idle-adjustment screw. Twist the idle screw clockwise two full turns with a Phillips-head screwdriver.
Start your motorcycle. The engine will sound like it is revving very fast for simply sitting in the driveway, this is because of the adjustment you just made to the idle screw. Allow the bike to run for ten minutes so that the engine warms up to normal running temperature.
Leave the bike running and twist the idle screw slowly counterclockwise with your screwdriver. You will hear the engine noise get softer and fall in pitch as it begins to run at a more manageable idle speed. Keep twisting the screw counterclockwise until you hear the engine sputter or begin to die.
Twist the idle screw clockwise one-half turn. Listen to the engine for one full minute. If you hear it sputter or die again in this period of time, turn the screw another half turn clockwise. When you are done, take the bike out for a test ride. It should be quick and responsive, and should return to this normal idle speed whenever you come to a stop or pull in the clutch.