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Old 04-23-2010, 10:56 PM
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curb idle speed problem

Hopefully someone can help me i have a weird problem i took my car out today for the first time in a while it ran mint with one exception...
before i left the house i brought the idle speed down to 950 to 1000 from about 1500 and after driving for a while it seemed to have raised itself back up keep in mind the engine was warm when i set it the first time ok well i reset it like 4 times and it kept raising back up i cant figure it out how could a curb idle screw be moving
its a holley 4150 750 dp thanks in advance for your help

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Old 04-23-2010, 11:12 PM
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First, check the mechanical advance in the distributor, if it`s sticking open or frozen in place it`ll cause a issue like you discribe. If it checks out okay, next is check it for vacuum leaks. Last is check the idle circuits.
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Old 04-23-2010, 11:34 PM
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Might need a new or stronger throttle return spring also.
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Old 04-24-2010, 10:17 AM
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i think it may be a small vacuum leak because i was driving the car for a while and after some time i noticed my vacuum gauge was reading 6-7 in hg instead of the 16 that it normally idles at. when i noticed it i just assumed the gauge was inaccurate because the engine ran fine(no backfires) and it didnt sound any different other than the high idling
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Old 04-24-2010, 10:41 AM
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could it still be a vacuum leak enough though it didnt raise back up instantaneously after i reset it back down it stayed for a couple minutes i dont notice it climbing back up til i stop and idle again
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Old 04-24-2010, 10:53 AM
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I wonder if your floats are sticking somewhat in the carb... I've had that happen before but mine would stick wide open and flood out the motor if I stepped on it more than half throttle and would let off the gass. My carb was a 750 Holley dp chokeless race carb. I had noticeable fuel puddles standing on top the throttle plates... fun!
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Old 04-24-2010, 11:22 AM
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thats a good point but i dont think it is the floats because it doesnt flood after letting off the throttle
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Old 04-24-2010, 11:33 AM
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mine was pretty extreme... when you readjust the carb repeatedly, do you find that the set screws are ending up in the same place you initially started or in an entirely different place each time?
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Old 04-24-2010, 11:42 AM
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Check the fit of the throttle blade shafts in the main body. When these get
worn and sloppy the throttle doesn't always return to the same place, in
addition this sloppy fit creates a inconsistent vacuum leak.
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Old 04-24-2010, 11:56 AM
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First, be sure the throttle linkage is sitting on the curb idle screw. If it's shy of resting on the screw and it's not being held open by the fast idle cam on the choke, the linkage is too stiff and/or you need a stiffer spring.

Otherwise, I'd also suspect a vacuum leak, could be the tranny modulator, PB booster, or anything that uses engine vacuum and the hoses and fittings that connect to them.

I wouldn't expect a stuck mechanical advance to cause a loss of vacuum, if not the same, it could be higher. A stuck advance could, however cause an increase in idle speed.
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Old 04-24-2010, 12:31 PM
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One thing I do a few times a year is lubricate everything that moves on the carburetor with penetrating oil spray. The red squirt tube puts the spray right where it's needed. Shoot the throttle shafts, accel. pump arms, secondary throttle linkage and everything else that moves. It really helps smooth operation of the holley. If you can do it while the engine is running, you can find vacuum leaks also. The idle will drop with spray oil instead of speeding up using flammable starting fluid or carb cleaner.

Now would be a good time to do it being it's been sitting for a while.
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Old 04-24-2010, 12:35 PM
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Another possibility is the secondary throttle position, make sure the return
spring located around its shaft is working correctly and the secondary throttle
arm bottoms out on it's adjustment screw (left side of carb).
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Old 04-24-2010, 02:00 PM
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Pupsvette76,

I would check the distributor hold down. I think the engine is advancing the timing and that is why the idle speed keeps going up. Grab it and see if you can turn it without loosening the hold down.

Scholman
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Old 04-24-2010, 10:12 PM
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turns out it was a vacuum leak from the tee going to my boost gauge thank you guys for your help
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Old 04-25-2010, 06:48 AM
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Vacuum leak

Boost gauge?
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