#1 What kind of bike? Particularly, is it SOHC or DOHC...because what I am about to say, I know applies to SOHC, but maybe not to DOHC.
#2 Are your carbs clean? I mean clean, clean, clean? (I say that because about 3 carb cleanings or more are needed to get them clean). It's no use trying to dial in idle if the carbs are dirty.
#3 What is the idle mixture set at? If the idle mixture is too lean, you will have a high idle and/or a hanging idle meaning it may start at a proper idle, but not return after being revved. On a SOHC with stock jetting, try 3 turns out of the idle mixture screws to start.
#4 After cleaning the carbs and adjusting the idle mixture, sync the carbs. Do it when the engine is warm, but don't let the bike idle forever. Air cooled bikes don't like just sitting still and running. Adjust the idle to 1200 rpm warm. That's the speed, not 800, not 1000- it's 1200 rpm (OK, 1250 if you just set it halfway between the 1000 and 1500). Then sync the carbs using a manometer or carb sticks, or gauges, whatever.
#5 If, after syncing the carbs, the idle speed changes, adjust it back to 1200 rpm.
#6 If the idle still hangs or creeps up as the bike gets warmer, try turning the idle mixture screws out in 1/2 turn increments, but don't go past 4 turns out.
#7 If that doesn't fix it, you probably have an air leak or your carb is still dirty in the idle circuit. Check for air leaks by spraying starting fluid or some such around the carbs at idle to see if the idle speeds up. Pay particular attention to the throttle shaft ends and the intake boots. If you find a leak, fix it. If you don't clean your carbs...again.
- This is all assuming a close to stock intake and exhaust system. If you, for some reason, prefer noise and therefore "opened up" the intake or exhaust tract....why didn't you tell me earlier? I would have told you to clean your carbs and up your idle jet size before trying any of this stuff.
