350 Backfiring problem

[my78chevyc10]

Hello, my 1978 Chevy C10 is my daily driver to high school and it has a Chevy 350 5.7l  is a pretty bare block with little electronics but it has headers and an Edelbrock intake and carb and a Delco HEI Distributor. The backfire is through the exhaust not the carb and happen when the throttle is opened up from idle (roughy 800 rpms) This started to happen after I changed the plugs, plug wires and removed the header to get the plugs. Any ideas on how to fix it would be great! Thanks.

[TexasRed]

Double check the plug wires to make sure they are tight on the plugs and distributor and that you haven't crossed any. Also make sure there are no exhaust leaks. But I'm guessing some plug wires are crossed.

[ITSANSS]

Probably the most likely cause is a crossed spark plug wire.

Have you triple checked that the spark plug wires are to the correct spark plugs?

Good luck

edit: TexasRed, ya' beat me to it.  

[78 Chevyrado]

Could be a header leak.  Did you put a new gasket on the header?   may also be bad timing.   could you have moved the distributor while doing the plug wires?   are all the plugs in there tight?  i have left one loose before
and are all the plug wires loosly installed (as in none of them pulled tight anywhere)  and did you stretch any of them at all?  sometimes that will break the conductor while the insulator looks good.

An exhaust backfire is either a leak:  it lets the cylinder suck air back into the cylinder causing a lean condition.

or a cylinder not firing all the time or at the wrong time:  unburnt fuel builds up in the pipe and then backfires when the stuff in the pipe actually ignites.

[my78chevyc10]

Thanks for all the quick replies guys. I checked my timing order and even checked that all my plug wires were snug and all seems to be okay. These plugs have a larger gap than before with a .45 gap whereas my others were .40 which I've read runs fine with an HEI distributor. I have new exhaust gaskets on the way and am hoping that is the issue. When going to a larger gap is it necessary to rotate the distributor any? Thanks.

[elixeric10]

well a easy  way to test this out is to put your old plugs back in if that dont work try messing with your timing .. just something i would try

[motion427]

You might have a bad plug,if everything else is good.
Somebody might have dropped the plug on the floor or something and cracked it.

Since you have the old ones,could try them again.Maybe just do one side at a time.

[Grim 82]

When going to a larger gap is it necessary to rotate the distributor any?

No. Wouldn't hurt to slowly go back to what was there before the problem and see if you can track it down. You can never visually inspect a plug wire for conductivity.

[my78chevyc10]

Well I advanced the timing on the distributor which made it run a lot smoother for some reason with no knocking but still afterfiring. It looks as if some black is starting to form around cylinder 2 exhaust header, I guess it's good thing I ordered those gaskets. I would love to try my old plugs, but I made the stupid/novice choice of throwing them away...  If it is a header leak causing the problem it still doesn't make sense that the fuel is making it's way through the exhaust and exploding. Thanks.

[Grim 82]

If it is a header leak causing the problem it still doesn't make sense that the fuel is making it's way through the exhaust and exploding. Thanks.

It makes sense if that cylinder isn't getting spark to burn the fuel because of a bad plug or wire....

[TexasRed]

Actually, in most exhaust there's enough hydrocarbons to keep combusting. That's what the air pump does, it pumps air into the exhaust manifolds and the gas still there burns a little more. High heat helps there too. So, it sounds like maybe it's the gaskets.

[Grim 82]

Yes, replace the gaskets for sure but if that doesn't do it look back at what you swapped out at the time when the problems started.

[Irish_Alley]

Get them aluminum ones from autozone in their "performance" section if a bolt backs out with the paper ones the will blow threw it and new gaskets are needed with aluminum ones you can reuse them and if a bolt backs out it wont blow out just really tighten the bolt

[my78chevyc10]

Well guys I replaced my exhaust gaskets and torqued down the header correctly and still the same thing. I guess it's time to go buy some new plugs and try my old wires that I actually kept. I really hope this solves my issue... When I pulled the header off, it looked like my cylinder 2 was kinda oily/wet I don't know exactly what that means but I did notice that.

[Grim 82]

What does the plug look like in that cylinder?