400 Chevy question

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Jesse80

Not-quite-so-new-guy
May 12, 2018
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I’m working on my dads 79 k20 plow truck. I built the engine 15+ years ago and slowly been getting it in better shape for him.
I replaced the stock exhaust manifolds with headers and new gaskets (that passenger side had no gasket at all), now it pops/ backfires. I have determined it’s the number 2 cylinder by removing the plug wire from the distributor cap.
My question is what is better for the engine. Leaving the spark plug wire hooked up and backfireing or unhooking it so it doesn’t?
He doesn’t want to rebuild the head cuz the lower end is probly in need of rings and an over haul too. Since he is paying for all the parts and work it’s his choice.
Thanks for the info.
 

bob64

G-Body Guru
Mar 30, 2017
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Neither!! Wire off all the raw gas washes the cylinder an goes to the bottom end. If it's just a exhaust backfire there is a valve problem which could end up destroying it all. Could be a simple bad lifter to a burn or bent valve.

So pick your poison.
 
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Jesse80

Not-quite-so-new-guy
May 12, 2018
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Thanks. I’ve told him I want to pull the head but he’s undecided at the moment.
 

ssn696

Living in the Past
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Jul 19, 2009
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Save that 400 for your hot rod. The finite number of them left is dwindling. Drop in a crate 350 into Pop's truck for $1800 and keep driving that survivor.
 
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lilbowtie

Comic Book Super Hero
Jan 7, 2006
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Compression check it to see if you have a problem first. Is the poping under the hood?? You could have a gasket problem.
As far as 400's I haven't had any luck with them - once you get around 500 HP they are prone to cracking.
 
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Texas82GP

Just-a-worm
Apr 3, 2015
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Did it run fine with the headers for any length of time and then develop this backfiring problem?
 

Jesse80

Not-quite-so-new-guy
May 12, 2018
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Thanks. I’ve told him I want to pull the head but he’s undecided at the moment.
It seemed to be running fine other then a massive exhaust leak, no popping prior to the headers. My thought is that after the headers and gaskets I sealed everything up instead of letting it leak out. The stock manifolds did not have any gasket on the pass side, it was metal on metal.
 

Jesse80

Not-quite-so-new-guy
May 12, 2018
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No it did not run fine for any length of time after the headers.
 

bracketchev1221

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Jan 18, 2018
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If it ran fine before the headers what makes you think the head has to come off? I would be checking for broken plug or plug wires.
 
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