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What could be the cause of engine run on after the ignition is turned off? My son just put a Holley 750 carbuerator and a new Mallory distiubutor on his 72 Mach I. He is getting the car ready for a fun drag race day in May.
 
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The problem is called diesaling. The engine is still detanating when shut off. Not really sure why though. Anyone know???

65 coupe 302
(rebuilding a 351W)

I'd rather push a Ford than drive a Chevy!!!
 
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My husband check the timing and he has the idle set at 1000 rpm like it says on the sticker on the car... but we still have this problem.
 

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1000 is to high. Back it down in 1/4 turn steps and try again. Is the car a auto.
or manual trans?

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Chevy drivers!!
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Hi,
Post the engine/trans combo so we can understand your situation better.

Generally, unless you have a fairly radical racing engine, you should be able to idle the engine at 750-800 rpm no problem...maybe 700 in gear with a C4...

The engine in my race car, for example, idles at around 950 rpm in neutral and makes nearly twice the HP of a stocker....

The number your husband read may have been for the stock emissions carb which had a idle cutoff solenoid which closed the throttle when you shut off the engine...

Please see this thread for some more info on running on and the causes...

Good luck!

Pat
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If you're engine's fairly fresh and clean running, ignore the following. However, if it's got a few miles on it, carbon deposits can do this. A carbon deposit in the cylinder can act like a deisel's glow plug (heats up during compression) and ignite the fuel after ignition cut-off, causing the "run-on". I had this problem with my 71.

Are the spark plugs pretty fresh? That's one potential area of carbon build-up. Valves are another potential area. If this is the case, true clean-up involves popping off the heads. I've seen fuel additives and other quick-fixes that are supposed clean this up. I have no idea if they work.

Good luck

DanM
66 Coupe, check her out at http://www.66CoupeNW.stangnet.com

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Mine would do this pretty often and it went away completely when in my battle against the dreaded ping I dropped the tmp range down on the spark plugs, both the ping and dieseling gone forever. It is like the same as the carbon deposits, you have a hot spot in the combustion chamber somewhere alowing the engine to run a little, my case the plugs, yours could be somthing else though.

69 Sportsroof, 351C, 4spd toploader, Restomod BOSS 429 clone - the one and only (?) 1969 BOSS 351

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Another thought. Depending on your compression you could need more octane in your gas. Having less octane than required by your engine will also cause it to diesel, sometimes bumping the timing back a touch will get you aound this (like when you deliberately have run higher octane and advanced the timing before and now want to go back to less octane).

69 Sportsroof, 351C, 4spd toploader, Restomod BOSS 429 clone - the one and only (?) 1969 BOSS 351

http://home.swbell.net/boss351/mustang/boss1.jpg
http://stangnet.cardomain.com/user/adamg69
 
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Discussion Starter #10
The engine is a 351Q and it's a 4 speed transmission. The engine has a edlebrock high rise manifold and I believe a quarter race cam. That is all I know....
 
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