Unless you have the carb manual, don't mess with the idle screws on a Mikuni.

The problem sounds most like a vacuum leak somewhere under the throttle plate, like a cracked or loose vac hose or brake booster hose. You get a lean out with that, too, which heats up the carbon deposits in the combustion chamber to DAMN HOT, like hot enough to act as a glow plug and keep the engine dieseling. Look for leakies with a piece of hose in your ear. You'll hear the whistle at the leak.

IIRC, the fast idle thing has a vac control, too, in the dash pot on the back dside of the carb.