• When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.

Backfire thru exhaust at idle

Could a weak negative battery ground be at fault as well?
Just took these pics a few minutes ago, in for the night now.



That photo is not a 69 quarter and that ground is not good,
Elaborating more...
I built the motor myself, broke in perfectly for 25 minutes with the high zinc Valvoline 20-50 break in oil (still in engine). I have run it since the break in approx. 2 months ago every weekend for about 10-15 min. with no problems. Idle, high rev, burping it all went fine. Very strong running in my opinion. Then, about 3 weeks ago, after I hooked up my old school SW gages under dash and connected cig lighter (which I don't think had an effect) but anyways, the next start up the following weekend, she started banging at idle after warming up.
I took all plugs out, they seemed pretty wet. I cleaned all with sand paper, replaced and it ran great even at idle for about 13 min. then the popping started again.
More background, is I done everything on this car, every nut and bolt. Welded my own exhaust, built entire motor, and all assembly as well.
I think you just landed the key - plugs! Had a really stout built 440 some years back that would pop at idle when the plugs got fouled. Ran fine with some rpms but popped like crazy at idle.
LMAO.. this is funny. On Friday I took the headers off the roadrunner and replaced them with HP manifolds and TTI down pipes. After I installed them and installed new plugs as well. I noticed a popping every now and then when at idle. I was like WTF? Well, upon looking further into it I noticed that I had 2 and 4 swapped on the plug wires. DERP... Swapped them around and its running great now. Double check your firing order.


Driver side front to back

Passenger side front to back...

Counter clockwise rotation
Late to the party as usual....
Your headers may be the cause of
the backfire issue. 2" header tubes
are rarely used in hp ratings less
than 500 on a 440.
Too large a diameter and the inertia of the incoming gas will be insufficient to scavenge the cylinder of exhaust gas. (unspent fuel in
the header).
You may be pushing unspent exhaust
gasses towards a hot header (now
red hot) with them acting like
a spark plug. Your backfire/popping
may be occurring in the header tube.
I'm thinking you need to increase
your exhaust flow rate. This problem
is magnified if your headers aren't
tuned as flow rates can vary.
Had the same problem on a newly
rebuilt 440 and had a set of headers
built custom, tuned, and 1-7/8 ID.
No backfire after warm up.
I know....just what you need...
something else to check out.
Last edited:
Thanks for the advise 1stMP. Yes, the 2" headers are kind of a pain. They are so close to plugs, wires, power steering, and mini starter. I may change out to a little smaller in future. I'm running heat wraps on everything. But, combined with the 3" exhaust straight back with glass packs dumping at axle, it does sound good too me and probably not change this summer. Maybe over winter I might. As for the popping, it has not come back once I changed to a power master alternator with the built in regulator. It seems Dadsby was correct in that a single wire 1969 alternator would not work correctly with a post 70 voltage regulator. Have not had a problem since at idle.
Auto Transport Service