400 Chevy question

Jesse80

Jesse80

Not-quite-so-new-guy
May 12, 2018
43
15
8
#1
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

bob64

G-Body Guru
Mar 30, 2017
585
317
63
Niagara Falls, Canada
#2
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.
 
Jesse80

Jesse80

Not-quite-so-new-guy
May 12, 2018
43
15
8
#3
Thanks. I’ve told him I want to pull the head but he’s undecided at the moment.
 
ssn696

ssn696

Comic Book Super Hero
Jul 19, 2009
3,081
1,015
113
New Mexico
#4
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.
 
lilbowtie

lilbowtie

Royal Smart Person
Jan 7, 2006
2,157
1,090
113
Canton Mi
#5
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.
 
B

bracketchev1221

Greasemonkey
Jan 18, 2018
219
129
43
#6
Was it running fine before you put the headers on?
 
Texas82GP

Texas82GP

Comic Book Super Hero
Supporting Member
Apr 3, 2015
4,355
3,528
113
Spring, Texas
#7
Did it run fine with the headers for any length of time and then develop this backfiring problem?
 
Jesse80

Jesse80

Not-quite-so-new-guy
May 12, 2018
43
15
8
#8
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

Jesse80

Not-quite-so-new-guy
May 12, 2018
43
15
8
#9
No it did not run fine for any length of time after the headers.
 
B

bracketchev1221

Greasemonkey
Jan 18, 2018
219
129
43
#10
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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