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Why does my idle RPM gradually increase?

  • Thread starter Thread starter Anonymous
  • Start date Start date
A

Anonymous

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I recently cleaned the carbs on my '83 GS550L. It used to stall and now it doesn't, so that's good. I notice now though that, as the engine warms up, the idle gradually increases from 1500 RPM to about 3000 or more RPM. After riding for a while, I can sit at a red traffic light and hear the engine begin to race. The idle speed will increase right as I'm sitting there.

Any idea what might be causing this?
 
One thing is to make sure you have a little slack in the throttle cable.
 
True. Check the vacuum line for the petcock coming from one of the carbs. If that's okay spray some WD-40 around all the intake plumbing and see if the rpm changes. If so it's an intake leak.
 
You have an intake leak. First look at the carbs and make sure they "popped" fully into place in the manifolds and filter boots. Then make sure all the clamps are tight where the carbs meet the manifolds and also the filter side. If you still have the problem, your manifold o-ring(s) are leaking. You can spray a fine mist of water on the manifolds to check for leaks. The rpm's will drop for a moment when the water hits. However, this test does not always show the leak because it's difficult too reach all sides of all manifolds.
If it's the o-rings, change all 4. Put some hi-temp' bearing grease on them to help them last and torque to 6 ft/lb. Over tightening just crushes them and shortens their service life.
 
Vacuum leak.

Vacuum leak.

I agree that the problem of high rpm at idle after the engine is hot is a result of a vacuum leak. I had the same problem until I replaced my intake manifold boot O-rings.
 
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