exhaust backfire causes 72 firebird 400
 

started getting exhaust backfire's recently, very loud pops. does it on intial start up when cold and does not do it once engine is warmed up.Changed normal gas station last 2 tanks. running quadrajet. could it be vac leak, what else can i look for before i blow my mufflers apart thks.

Intermittent pop in the exhaust usually means a failing condenser, assuming you don't have HEI. If you do have an HEI, it could be a module problem.

The problem is an indication of an electrical problem where it loses fire momentarily.

running pertronix in original dist

Too rich at idle or mild flooding at start-up [?]. Leading to excess fuel getting in exhaust and igniting. How do the plugs look ?

Jim

The exh system on a cold motor that was just started up does not have enough heat to light off extra un-burned fuel!
I would pull the Q-jet apart and I bet you will find that the primary power piston it somewhat hung up and not providing enough enrichment, or you have a vacumm leak some where.
Is the motor hard to re-start when fully hot, as in cranking slow?

Do you have to pat the gas a lot to get it to start cold? If you fill the exhaust with unburned vapor and then throw an exhaust flame into that, it'll bang pretty good. Also verify base timing hasn't slipped and become retarded.

car starts just fine when hot and cold needs a few pumps of the gas but same as its always been, have not had a lot of time to track things down but so far i found the dist advance bushing had fell out-using piece rubber hose-but that should not be the problem and of course i pull my timing light out and it does not work so i need to pick up another one to verify timing.

went and got a new timing light hooked it up and guess what #1 cyl is firing intermittently, put new plug in and looks good now, hope this was my problem

That should do it unless it was a bad cap or plug wire .