750 Holley Idles too High, Sometimes - Team Camaro Tech
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post #1 of 8 (permalink) Old Sep 14th, 05, 10:17 AM Thread Starter
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Cameron
 
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750 Holley Idles too High, Sometimes

I have a 750 vaccuum secondary Holley on my 400sb. It used to work fine, but the last couple of years, it has idled too high when warm. The problem gets worse as it gets colder outside. It will idle anywhere from 1200-1800 RPM, but if you let it idle long enough, it will usually make its way down to 900 RPM, but it may take a minute or sometimes more. I have checked for binding throttle linkage several times and it isn't binding up or sticking. I have also adjusted the idle mixture several times with no apparent help. Floats are set correctly. Could it be a power valve problem? That doesn't seem likely to me. I am usually a decent carburetor guy, but this one has me baffled. Any ideas?

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post #2 of 8 (permalink) Old Sep 14th, 05, 10:59 AM
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Re: 750 Holley Idles too High, Sometimes

Do you have a choke? If so, what kind?

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post #3 of 8 (permalink) Old Sep 14th, 05, 11:21 AM
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Re: 750 Holley Idles too High, Sometimes

Could be the advance in the distributor sticking , either mechanical or vaccum.

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post #4 of 8 (permalink) Old Sep 14th, 05, 11:49 AM
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Re: 750 Holley Idles too High, Sometimes

Quote:
Originally Posted by SIDEWAYS
Do you have a choke? If so, what kind?
Ditto. It sounds like it may be a choke issue.
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post #5 of 8 (permalink) Old Sep 14th, 05, 02:56 PM
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Re: 750 Holley Idles too High, Sometimes

If power valve was bad, it would be dumping fuel and "loading up," lots of black smoke.

If floats are set too high, fuel will dribble from the venturi boosters.
Clogged idle air bleeds will show the same.
As suggested before, sticking advance weights will advance timing and idle speed goes up.
If manual or electric choke problem, fast idle cam could be sticking.
If electric choke, heater coil may be broken or a bad electrical circuit all the way to ground.
A vacuum leak maybe between the carb and intake or intake and head(s).

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post #6 of 8 (permalink) Old Sep 14th, 05, 04:51 PM
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Re: 750 Holley Idles too High, Sometimes

And one last common problem with the Holleys that may be your issue-the bushings in the throttle plates get worn and then allow a vacuum leak that can manifest itself intermittently. With the car idling, try spraying WD40 directly onto the ends of the throttle shafts to see if the engine RPM changes. I- have had SEVERAL base plates that needed to be re-bushed. When spraying the WD 40 try and rock the shafts within the bore to see if moving them causes additional clearance.

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post #7 of 8 (permalink) Old Sep 14th, 05, 06:22 PM Thread Starter
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Re: 750 Holley Idles too High, Sometimes

The choke is electric and it is functioning properly. I checked that too. I bought this carb new a few years ago and it has 10k-15k miles on it so I would hope that the throttle shafts aren't worn, but I will check. I used to think that it was an intake leak too, but I have had three different sets of intake gaskets on, with no difference. The advance weights sticking is possible and I never would have thought of that. I'll look into that sometime this weekend. I have a Mallory mechanical advance Unilite distributor, if that makes any difference. I have had it for about 4 years. It probably has 20k miles on it.

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post #8 of 8 (permalink) Old Sep 14th, 05, 07:08 PM
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Re: 750 Holley Idles too High, Sometimes

I have a 770 Avenger, and when I got it, it was doing what your does. Only difference is sometimes it would idle at 1200 instead of ~750. I had to rotate the choke housing clockwise slighlty so the choke flap would stay opened tightly. Turns out their wasn't enugh tension to keep the flap fully opened - sometimes the flap would close slightly and the fast idle mechanism would engage. If I stabbed the throttle a couple of times, th rpms would come down.

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