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GMC V6 and V12 Engines Engine repair and rebuilding

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  #1  
Old February 27th, 2017, 09:32 PM
jagarra jagarra is offline
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Default Truck won't idle

I must admit I have not driven my 62 GMC with the Holley 500 cfm carburetor that much in the past year. The last time is ran normal was my trip to the dump in mid September. When my cousin and I had an errand to run last month it just did not want to run at low speed at all. It flat refused to idle at all, dying at every stop light or when the speed got too low or throttle position got too low. I figure my idle circuit may be blocked with crud from that oxygenated fuel, but the last time I filled it's tank was with pure gas no ethanol. Any ideas??
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  #2  
Old February 28th, 2017, 12:21 PM
turbobill turbobill is offline
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Default Re: Truck won't idle

Unless you flushed the ethanol mixed gasoline completely from the entire fuel system, there was/is still some in your truck. Put simply, the stuff is nasty. I don't even run the stuff in my newer vehicles.

If an idle passage is blocked, sometimes I've had luck with revving the engine up, then putting the choke fully on until the engine almost stalls. I may repeat this several times until the blockage is sucked out of whatever passage it is in.
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Old February 28th, 2017, 09:36 PM
George Bongert George Bongert is offline
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Default Re: Truck won't idle

Greetings jagarra!

Disassemble the carb and soak it in Acetone and blow out with compressed air. Acetone will get the varnish and other gunk out of the carb that commercial carb cleaner won't get!
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Old February 28th, 2017, 10:41 PM
AZKen AZKen is online now
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Default Re: Truck won't idle

It could have developed a vacuum leak. Check all rubber hoses, caps, plugs, carb bolts, gaskets & intake ports for leaks. The choke trick does work. You can use your hand or the actual choke.
If not vacuum (you can test with carb cleaner or propane), then the carb probably.

There a slight chance that the gas cap is not venting but that should show up at high rev also.

Last edited by AZKen; March 1st, 2017 at 12:39 AM.
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Old March 1st, 2017, 10:06 PM
jagarra jagarra is offline
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Default Re: Truck won't idle

The winner is AZken. After I pulled the carb and flushed out the idle passages the real problem came to light. All three of the caps blocking off vacuum ports on the carburetor were either cracked and exposing air or the end completely gone. These are the ones that come with the Holley from new, lasted less than a year, maybe replacing them with better quality when installed could save you some headaches later. After I got it running and the idle adjusted I found that little black plate that goes in the carburetor in the with the float. maybe will replace tomorrow.
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Old March 1st, 2017, 11:25 PM
AZKen AZKen is online now
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Default Re: Truck won't idle

"Lucky" winner. Good job Jag.
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  #7  
Old March 10th, 2017, 06:37 PM
jrmunn jrmunn is online now
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Default Re: Truck won't idle

FYI - Glad to hear that you were able to find the idle problem. Also, for those of you with the Holley 500, a backfire can push off a loose fitting cap from the large vacuum line port at the back of the carburetor, which will cause the engine to die at idle. This happened to me after I killed the engine by letting the clutch out with the emergency brake on.

JRMunn
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