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Very high idle after bike gets hot

Posted: Mon May 08, 2017 10:49 am
by Crimson
Anyone have any ideas what could cause this? When the bike is cold, it needs choke pulled out to start. This is normal, I know. The choke makes the bike rev up to 3k rpm. Also normal. Then, when the choke is pushed in, the bike idles at just under 1k rpm. It's possible to adjust the idle speed at this point using the big idle adjustment knob behind the carbs. All well and good!

The funny business starts when the bike has been fully warmed up. It won't exhibit any strange behaviour if you leave it idling for 5 to 10 minutes. The idle will go up to about 1.3k rpm. But if you then take it for a ride, in stop and start traffic, interspersed by fast, short countryside blasts, the engine gets hot and the idle sits at 2.5k rpm, threatening to creep up even higher. At this point, you could fully unscrew the idle adjustment knob. It won't do anything at all. Oh, and I've checked the throttle cable. It isn't catching or sticking. I've even removed the 'push' side throttle cable, to reduce the load on the spring, so it is working and the throttle does snap back properly.

Really strange. All I've done with it is flush the carbs out with carb cleaner (rough and ready chemical method) and crudely balanced 1 and 2 vs 3 and 4 using a pair of vacuum gauges. I've squirted carb cleaner all around the boots of the carbs, and around where they connect to the air filter. No changes in revs, which means no air leaks. What could cause this?

Thanks