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Random Engine Popping/Backfire?


Active Member
Local time
1:01 PM
Sep 19, 2011
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Las Vegas
Not sure what's causing the random pop/backfire? Any help is appreciated! Here's the scenario:
440 block stroked to a 512
Full MSD Ignition system. 6AL2 box, billet distributer, master blaster coil, 8.5 cables
Quick Fuel 850 double pumper carb

While racing at Pomona last weekend I was 3/4 track WOT and a loud pop occurred. The car lost power temporarily and nosed over. Pedal power was still there as i crossed the finish and drove the car back to my pit. While loading the car onto the trailer I noticed very random popping from the exhaust. When I got the car home and while taking it off the trailer the same random popping was occurring. I got the car into the shop, pulled the distributer off to look for anything obvious, and it looked brand new. I then checked for voltage being supplied understanding that msd boxes want 12 volts, and noticed my battery was low so I charged that up to 100%, started the car and the popping still existed. I started the car in the dark to look for any wire arching and saw nothing. I'm prepared to pull the carburetor and inspect the power valve? I plan on pulling the plugs to inspect them as well. I also wonder if there's a bad vacuum leak around the intake or headers. I feel like I'm just guessing at this point though. Has anyone experienced this? I appreciate the help!!
Popping through the exhaust is often a matter of the unburned exhaust gases igniting in the pipes or mufflers.
I suspect an ignition issue.
I had a similar issue with a car years ago....I was doing a stunt and the distributor cap in a 318 made contact with the firewall, knocking it off the distributor enough to make the rotor tip hit several terminals. The rotor tip was knocked loose and the car ran terribly.
I'll join the "valves" crowd (I hope I'm wrong). Had an engine that would pop in the exhaust on a seemingly random fashion. Found out, when I took the heads off, that one had an exhaust valve slightly open. That valve closed when I nudged it a bit. The guides had not been properly reamed. That engine had kinda light springs.
Agree, check springs/prods. If a hyd cam, might have pumped up a lifter...

Also, check the air gap for the dist p/up. If the reluctor is touching on one of more cyls, you get no spark on that cyl.
Push rods good
spring pressure good
valves good
drivetrain in general good
two bad plug wires replaced all of them to be safe
rebuilt carb and found a hole in the accelerator pump diaphragm
replaced the valley pan and installed new paper gaskets. I was installing a new Trick Flow high rise intake manifold and spacer
No vaccum leaks to be found

The random popping has ceased! Now i'm fighting a lean condition on startup. Lots of backfire during initial start. Flame is chasing the fuel back up the carb. Not sure if larger squirters would help this? I have to pump the gas a lot to get enough fuel in for it to start. I expected an adjustment with the new intake. Just trying to dial it all in now.

Not sure what exactly caused and then fixed the random popping, but something did it? I know it's frustrating to make multiple changes at once, but I didn't want to keep pulling things apart.

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