Can someone give me some ideas on how to chase down a problem with my car backfiring through the exhaust when I let off the gas? It's a 440 with a Holley chokeless main body carb and QFT metering blocks and bowls.

I thought it might be an exhaust leak at the collector gaskets, but I replaced them both to no change.

Assuming no exhaust leak, it would have to be carb related, right?

Thank you.

Greg

to lean on decel will cause it to do that, if you are saying it pops out of the exhaust

Every popping when let of I've work on and helped people solve has ended up being outside air suck into the exhaust
Gaskets, cracks, bent flanges and so on

My experience, too rich on deceleration (high vacuum pulling excessive fuel through the idle circuit) and exhaust leaks.

Try leaning your idle and plug your tailpipes to find the leaks.

Rude runner nailed it. It’s pig rich at idle and cruise and then on decel all that fuel goes off late and causes the back fire. I have never, even seen a header leak cause a backfire unless it pig rich.

Seen that happen with an intake manifold leak.

Okay, thanks everyone. I think I will pursue it initially as being too rich and go from there.

Tooooo lean can also cause this condition, my HD did it until I upped the pilot jet 1 size.

Backfire in the exhaust is from unburned fuel in the pipe. Yes, it can be from too lean but you'd probably have misfires and generally crappy running if that lean.

On a Harley, you'd never notice the poor performance.

And I bet the carb was dialed in rich to cover the vacuum leak.

Could have been. IIRC, it was from the valley side and was discovered by the oil tracks when the intake was removed for a camshaft change. But it's been a long time ago and the memory gets fuzzy.