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Idle reving up

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My bike start normal then it revs up to about 3/4 k when getting warm...

What could be the problem?

Its a 99 efi btw

 

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Vacuum leak?

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Posted (edited)

Sounds like the fast idle mechanism isn't releasing once the coolant starts to get warm.  There is a plunger, filled with wax, I believe that disengages the fast idle.  The wax expands with heat and moves the plunger.  I believe it's on the left side of the throttle bodies.

Edited by jon haney

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+1 on what Jon said.  Mine is failing in the other direction, idle drops really low (doesn't stall).  It's a shit design like the CCT.

 

Vacuum leak can cause some erratic idle, but I don't think it can send it up that high on this vehicle.

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Vacuum leak can cause erratic idle only on engines with feedback idle control speed, so it doesn't apply here.

 

Going back to OP, I think there are couple compounding issues here. I don't think wax stuck wide open would send idle speed that high. Most likely idle speed is set too high as well.

 

I would disconnect fast idle speed apparatus first and check idle speed after the engine is up to normal operating temps.

There could be vacuum leaks as well.

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It was like this last year as well

It went away eventually but back now..

The idle is set right.

I never tuched that knob..

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1 hour ago, Roar said:

It was like this last year as well

It went away eventually but back now..

The idle is set right.

I never tuched that knob..

 

You didn't quite catch what we were saying.  There is a wax-filled capsule that automatically opens the throttle when cold.  As the engine warms up, it slowly allows the throttle to close.  Cold engines need more throttle and a richer mixture.  When it's working well, you don't notice this because it works at the same rate as the change in temp-based idle speed.  My idle starts normal when cold, then drops a lot as the throttle closes too soon.  Yours seems to be doing the opposite.  I don't recall what it takes to remove that part, but if not hard, I'd start by doing that and see if the problem stops.

 

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Keep in mind that "fast idle" is the default position for that mechanism.  If you could remove the wax, it would idle fast all the time.

I agree with Carlos that it's not a great design.

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On 5/19/2020 at 8:23 AM, Roar said:

My bike start normal then it revs up to about 3/4 k when getting warm...

What could be the problem?

Its a 99 efi btw

 

What's the RPM at initial cold start and does it slowly steadily climb as the engine warms up, or suddenly go up to 3-4K?

 

The fast idle device is probably the answer, but could be other stuff.

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Just to eliminate one possibility (cable binding), trace your throttle cables and then rotate the forks lock-to-lock; there should be no change in the idle rpms. If there is, look for something binding/pinching/etc. the cables, or miss-adjusted cable slack.

 

If there's no fork movement when your idle fluctuates, it's more likely not a cable problem (but double check to be certain). 

 

Otherwise, the fast idle circuit and potential intake air leaks would merit close scrutiny.

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16 hours ago, superhawk996 said:

What's the RPM at initial cold start and does it slowly steadily climb as the engine warms up, or suddenly go up to 3-4K?

 

The fast idle device is probably the answer, but could be other stuff.

It slowly goes up..

 

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On 5/21/2020 at 3:17 PM, Roar said:

It slowly goes up..

 

I stand by my original diagnosis.  Keep in mind that lack of coolant flow to that mechanism can also cause the same symptoms.

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