Popping/backfire through exhaust on loaded downshift
I've done a lot of searching on this forum and others on this topic, and I know there have been a lot of answers, but I'm not sure I have the same issue. Or at least, others that I've read have not described the same symptoms.
I have a '68 Camaro with a pre-81 GM Goodwrench crate engine (350ci - made in Mexico type) with Hooker headers, Flowmaster exhaust (2.5"), an Edelbrock 1806 Carb, Mallory HEI (nothing fancy), mild cam, and Accel wires. Timing is set at 6* BTDC and have a strong spark. No discernible exhaust leaks. Muncie M-21 tranny.
I have strong, smooth acceleration under load, and smooth no-load deceleration (clutch in, load off engine - decelerates quietly and smoothly). However, if I downshift and decelerate under load (engine braking), I get all kinds of popping and backfiring through the exhaust.
This is a recent occurrence, and not associated with any engine work. I'm guessing that the solution is probably a simple one that's right under my nose, but I'm kinda at my wits end.