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GMC V6 and V12 Engines Engine repair and rebuilding

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  #1  
Old February 21st, 2024, 03:24 AM
gear jammer gear jammer is offline
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Default correct 478 spark plug

howdy
I've got a 478 in a bus. I rebuilt it years ago and have always had a dieseling problem when I shut it off. if you shut it off when it's cold or after start up it's OK. but if you let it run awhile it diesels. it's a 478 magnum. early no taper seat plugs and it's idling at 500 rpm. it's got ac r43xl plugs in it 3/4" reach
I haven't tried any other plugs but I read dieseling can be caused by too hot a plug
any help on what should be in there?
steve
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  #2  
Old February 21st, 2024, 04:11 PM
Prowbar Prowbar is offline
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Default Re: correct 478 spark plug

From my 1965 manual, the factory spec is a C42N plug, so a little colder. 3/4 reach is fine.
Haven't had any trouble with my 478 dieseling. Mine is a 62.
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  #3  
Old February 21st, 2024, 07:08 PM
gear jammer gear jammer is offline
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Default Re: correct 478 spark plug

thank you for that information. makes sense that's a range colder and would solve my problem
steve
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  #4  
Old February 24th, 2024, 08:05 PM
Prowbar Prowbar is offline
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Default Re: correct 478 spark plug

Good luck, hope it solves your problem.
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