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Join Date: Nov 2007
Location: Chandler, Arizona
I have had bikes where the idle was very erratic. It constantly went up and down. There are a bunch of possibilities. Jets could be clogged, float level could be off, it could be a vacuum leak, an intermittent fuel supply to the carbs, poor carb venting, carbs not synchronized, pilot screws not adjusted properly, a leak around or in the diaphragm, or even to much of a difference in compression between cylinders. It's rarely the ignition, but it's possible.
One of the neat things about carbs is that if everything is right, they can be tuned to achieve a dead stable idle. I have never seen a fuel injected engine (car or bike) with a nice smooth idle, and the throttle response is never as good either. When you rev a carbed engine and release the throttle, it drops back to idle instantly. When you rev a fuel injected engine, it usually takes 2-4 seconds before it comes all the way back down to idle.
I think a stock carbed engine in good mechanical condition should have a nice steady idle. I do have one engine (in a truck) that won't idle smoothly, but it has a long duration cam.
I am a motorcyclist, NOT a biker.
1997 Vulcan 750, purchased about a week ago
2006 Sportster 1200 Low
2013 Royal Enfield Bullet 500, converted to carb
2001 Yamaha XT225, heavily modified
2004 Honda Rebel 250
1979 Vespa P200E
2002 Vulcan 750 parts bike
1994 Yamaha XT225 parts bike