An engine that alternates between a higher than normal idle speed and idling very poorly or dying, either presently, or when the idle speed screw is backed off by some small amount from where it is now, is caused by poor cylinder sealing, significantly worn cam lobes, incorrect synchronization, and/or a grossly incorrect idle mixture (on one or more carburetor) in combination with the idle speed screw being screwed in too far in an attempt to get it to idle anyhow. A grossly incorrect idle mixture can be caused by an incorrect idle mixture adjustment, clogged pilot jet, incorrectly sized pilot jet (possibly from “cleaning” by poking things through), clogged pilot air bleed jet, incorrectly sized pilot air bleed jet, overflowing float bowls, vacuum leaks (check engine side carburetor boots, vacuum port caps, petcock vacuum hose, petcock diaphragm, throttle shaft seals, diaphragm cover vacuum passage o-rings, and fuel screw o-rings), and/or the cold start enrichment circuit being active when the engine is hot.
An engine that simply stays at a higher than normal rpm (i.e. does not alternate between a higher than normal idle speed and idling very poorly or dying) is caused by incorrect idle speed adjustment (for example if the idle speed is set without the engine being hot), incorrect synchronization adjustment, incorrect cable adjustment, sticky or binding cables, or by the fast idle portion of the cold start enrichment circuit being active when the engine is hot.
An engine that exhibits a changing idle speed as the handlebars are turned is caused by an incorrect cable adjustment.