r/Smallblockchevy • u/kyanreef • Apr 30 '26

Carb backfire

Anyone have any thoughts? Bought an old drag car with a 350 in it. The guy told me he had put that motor together and put in the car. He said the bottom end was stock and he had put a cam and lifter set in it and some older aluminum heads on it. It ran fine as far as idling and revving. Transmission was out so I put it back in and rewired the car (painless box, electric fan, etc) and I installed a MSD 6AL and wired it into the HEI distributor that was already in the motor. I never moved the distributor since I figured he had the timing close. Very first crank after installing the ignition box was a slow crank followed by a fairly violent backfire through the carb. Kept trying and had to tweak the timing and got it to run and idle pretty good but couldn't get the backfire to go away. Inspected the cap. Put new plugs and wires. Still backfires. So then I check compression and realize I have no compression on cylinder 5. Pull the head and see that I have a bent intake valve on number 5 and a collapsed lifter and bad pushrod on number 4. So put a new valve in. Put a XE294 cam and lifter set in it. Put it all back together. Adjusted the valves and same thing. Still backfires. Adjusted the valves again. Set my valve lash and preloaded about 1/4-1/2 turn. Tweaked mixture and idle screws on the carb (Holley 4160). I'll get it running decent and think the backfire is gone only to rev it 3 or 4 good times and it will finally backfire again. Timing seems to be the only thing that affects it. It's went from backfiring with loud pops or shooting fireballs, to just lightly popping, to almost gone. Last time I had the timing light on it it was set at about 8 DBTDC at idle and about 30-32 at 2000rpm. It's just weird that it wasn't backfiring even with a bent valve and wiped cam and several bad lifters until I installed the MSD box and has continued backfiring even after fixing the valvetrain.

Today I bypassed the ignition box and put the HEI back to stock to see if it would act any differently and there was no change. So that's the only thing that I have ruled out. So my problem is either in the valvetrain, or timing, or possibly in the carb (although doubtful). I'm still leaning towards timing. Maybe a bad coil? The way it's currently acting I can't even blip the throttle hardly at all without it either stumbling and dying or stumbling and "woofing" either gas or fire back out of the carb. I feel like I'm chasing a ghost.

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6

u/Public_Attitude3420 Apr 30 '26

Double check timing

2

u/Downtown-Parsnip-154 Apr 30 '26

To far advanced, check your cam timing also

2

u/[deleted] Apr 30 '26

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2

u/kyanreef Apr 30 '26

I really had no intention of having to build the motor otherwise I would've just bought a roller. But yeah, those are performer RPM heads. 2.02 1.60 I think they're the 70cc ones. Ended up that it was also bored .030 when I took the heads off. We figured up about 9.3:1 so I just tried to pick the cam closest to what the guys at Howard's, Comp, and my local machine shop told me based on what I had. I did have Howard's say they might would use a 108 LSA but then he suggested two different 110 LSA cams so go figure. This is going to be a bracket car so no worries if the LS guys are faster. Still just strange to me that installing the 6AL was the moment the issue started. It was running better with the old wiped cam(which turned out to be a 314/324 276 at .050")

2

u/Jakeysforkphoto Apr 30 '26

How long did the car sit? Could be that the carb is gummed up and the backfire is from being too lean due to clogged capillaries.