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Old 06-07-2004, 05:30 PM
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engine run-on

I have chevy 468 with run-on.Ive changed carbs., but no difference,had msd 6al,same.Went back to hei, but still the same. I have holly 750 on it. I tried a kill switch in-line to the distribuator. Please help!!!!!!
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Old 06-07-2004, 05:32 PM
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Does it run for ever or just a few seconds.

Dumb question but how hot is it runnig at operating temp. Or when it's doing this.
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Old 06-07-2004, 05:32 PM
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Question

on the carburetors that you're using, did they have idle solenoids, also what is your idle set too?
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Old 06-07-2004, 11:07 PM
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Your throttle plates are open too much when you shut off your ign. Thats where (I think) Lucid is going with his response. Too much air going into the engine after shut down will cause run on in most cases. Get the throttle plates to close completely by adjusting or you may need to add an idle speed solenoid.
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Old 06-08-2004, 12:27 AM
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run on

do you have an excess of carbon build up if u do try bringing up the idle 1500 2000 and pour a small amount of water down there it will bust the carbon off down side you could get a piece of it stuck in a valve seat
more practical try chevron gas products or techron good stuff good luck with that
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Old 06-08-2004, 03:03 AM
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water will help clean out the carbon

it does work, just bring the engine up to temp and pull off the air cleaner. with a bottle of water in one hand keep the engine running with the other on the carb linkage, pour in the water a little at a time till about 12oz are gone. Thast should remove any traces of carbon inside the combustion chamber, cleaning the valves and the top of the pistons.


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Old 06-08-2004, 07:09 AM
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won't stop

Sounds like carbon build up, after all, no spark, no fire
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Old 06-08-2004, 08:48 AM
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It's not a fix but, I found that it you kill the engine with the trans still in gear, it will not run-on.



C
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Old 06-08-2004, 09:21 AM
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What may also help to get rid of the carbon is to use a can of "top engine cleaner" before you do the water. The top engine cleaner will soften the carbon and make it easier for the water to do its job. Some people use Marvel Mystery Oil as a cleaner.
BTW - diesels run on run on. They stop the engine by shutting down the air. Same concept here, hot carbon and air will try to keep the engine running.
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Old 06-08-2004, 01:26 PM
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Run On

Though most likely one of the previous posts accounts for it, occasionally you can get a piece of head gasket material hanging in the combustion chamber which can also cause that.
Have you had any overheating problems?

BTW, be carefull with the water, it's possible to score a cylinder if you do too much too long.
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Old 06-08-2004, 01:55 PM
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Would having the timing too far advanced cause this also?


C
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Old 06-08-2004, 03:32 PM
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If the engine is running hot the fuel that is sucked into the cylinders will ignite just from the heat and no spark. You should let the people know that is trying to help you a little more info. Just by looking at the responses so far there is anything from one end of the spectrum to the other.
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Old 06-08-2004, 10:03 PM
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engine run-on

I can't shut it down in gear because its on a airboat.The engine runs 130 but gets to 160 if i travel for 10 miles or more. Thats holding at about 4200 rpms.The timeing at 38 degrees,but Ive retard it down to 28 but no change.What about carbon? How does it make it run on? I'll try that tomorrow with water.When it runs-on it is for about 15-30 seconds.If i start it up and just idle(600 rpms) and let it warm up it shuts off fine,but wonce ive driven it ,it will do it every time. thanks joe

One more thing.Ichanged carb from my truck thats the same size and works good on it , but it still no good on my boat. Is it runing to cool? I'm going to check out for carbon. Thanks for help!

Last edited by joe svoboda; 06-08-2004 at 10:03 PM.
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Old 06-08-2004, 10:12 PM
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I had the same deal with an old 350. I ran some stuff called Seafoam through it. Got it at Advance Auto. I just dumped it in the carb while it was running. It will smoke like a steam engine and try to stall, jst rev it up and dump it in. It seems to have worked. I changed the spark plugs to a colder heat range as well, just to be safe.

Larry
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Old 06-08-2004, 10:30 PM
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Change the thermostat or put one in if you don't have one, 130F is too cold for a liquid cooled engine. Likely liquid fuel is pooling in the intake and trickling into the chambers during shutdown causing your run-on condition.

Is the exhaust heat crossover in the intake still functional? Check to see if it's plugged.
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