Racing Idle!

[rmclaughlin]

Hey guys n gals. I have just bought a 79 Special. Got her purring recently and when at temperature when the clutch is engaged, the idle races upward. If I goose the throttle it come down(sometimes). I have read it could be an air leak around the intake or vacuum. The previous, previous owner took off the vacuum manifolds and petcocks and replaced them with non vacuum style. The previous owner put different exhaust on it. I have read that I may need to re-jet if the exhaust has been changed. If so, how to I determine which jets etc. Any advise is appreciated.
Thanks
Rob

1979 XS650 Special, aftermarket exhaust, mostly stock.

 

[xjwmx]

Look at the cable lever on the carb and make sure nothing is hanging. Try to push the lever down when the idle is up.

Also, if the idle is set too high to start with it can zoom up, I think due to the advance kicking in.

I like to use carb cleaner to test for leaks. It's not messy, in fact cleans stuff off, and isn't flammable. If you find a leak the speed will go down. If there's a leak big enough to cause this it should be pretty easy to find.

I had one bike where the exhaust valve was too tight and was leaking air during the intake stoke which caused something like what you describe. Adjusting the valve tappet fixed that.

 

[retiredgentleman]

Hey guys n gals. I have just bought a 79 Special. Got her purring recently and when at temperature when the clutch is engaged, the idle races upward. If I goose the throttle it come down(sometimes). I have read it could be an air leak around the intake or vacuum. The previous, previous owner took off the vacuum manifolds and petcocks and replaced them with non vacuum style. The previous owner put different exhaust on it. I have read that I may need to re-jet if the exhaust has been changed. If so, how to I determine which jets etc. Any advise is appreciated.
Thanks
Rob

1979 XS650 Special, aftermarket exhaust, mostly stock.

Could be any of the following, or a combination of them all:



Carbs, jet needle O-ring is worn out, passing extra unmetered fuel.

Air inleakage around the carb intake area.

ATU fly-weights sticking/ATU shaft not lubricated.

Carbs not sychronized. Its important that both butterflys are able to close fully, and are exactly in sync. as they open. Bench sync butterflys with a piece of paper first then use a manometer.

The carb mixture screws also need to adjusted correctly.

For carb jets, you need to read the "Carb Guide" that is in the "Tech " section.

 

[rmclaughlin]

Thanks All. I'll start with the simple stuff then move upward.
Rob

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[XSsesive50]

Just a question regarding carb holder leaks. When installing new holders, what is the recommended sealing method of the holder, gasket surfaces? Anyone use a sealer or just bolt them on dry?