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Discussion Starter · #1 ·
I have an idle problem. I have a 70 Chevelle with a Quad jet carb and 454 BB and an auto TH400 tran. I need to know how to adjust the idle/air mixture and set the idle speed correctly. I have heard to turn the idle/air mixture screws 2 turns and I have also heard to do it 4 turns. I have tried that and everything in between. I can get the car to idle at about 1500 rpm and put the car in drive and it drops to about 600 rpm like it is supposed to but it starts idling a little lower the longer it sits in drive. Also, while driving, it is surging a little like it is starving for fuel. I have changed the plugs and wires. Any other suggestions? It may not have anything to do with the carb because the carb has been running fine and it is only about 3 months old. I may be barking up the wrong tree by spending so much time on the carb. I don't see any obvious leaks. HELP!!!!
 

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66 El Camino 57 Chevy pickup 2004 Tahoe
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Sounds like you have a big vacuum leak or maybe the vacuum advance is hooked up to manifold vacuum instead of ported vacuum. When did this start happening? Has it always been like this?

Tom
 

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Ditto. Check for leaks. In fact, I recall to set idle circuit, you close the screws and then open them up 2 turns. The best way to set it finally, though, is via the use of a vacuum gauge. The gauge might also help you pinpoint a vaccum leak. Of course, I had more than one black ear by poking around a running engine with a piece of vacuum hose in my ear searching for that " big sucking sound".

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[This message has been edited by Gene Chas (edited 05-21-99).]
 

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The creepin up of the idle speed may be a leaky inlet needle which is allowing the fuel level to raise and richen the mixture. Does the exhaust smell richer as it slows down? As for the method of setting the idle, I prefer to look for lean rpm drop as indicated on a tachometer. Turn in a needle until the rpm drops noticeably and back it out until it is at the position that is just richer than the rpm drop point. It usually takes several rounds of this to get left and right needle set properly.
 

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Discussion Starter · #5 ·
It was running fine about a week ago. My wife was driving and she said it made a sound like a ball dropping down a hollow pipe and since then has ran rough. Since then, I changed plugs and wires because it did have a bad #1 plug wire but this seems to not have had that big of an effect on it. It cranks a lot better now but still surges and cant get the idle down.
 

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When the idle starts dropping off, try revving the engine a little, hold it at a fast idle for a few seconds. Then bring it back to idle. Does it idle normally for a little while before it starts dropping off again? If so, the problem could be a saturated float causing the engine to load up at idle. It will idle OK for a little while, load up, and revving the engine will clear it up by burning up the excess fuel. The problem will repeat itself when the engine is brought back to idle.
 

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Does your chevelle have a fuel pump with a return line? If your fuel pump has a provision for a return line and you have it plugged off, you are making the fuel pressure jump to about 17lbs of pressure. It
sound like it is flooding. If you have a cam
with a lot of duration and the engine has low
vacuum you have a problem with the power spring that activates the power piston. It works just like a power valve in a Holley.
 
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