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Does it pop out the exhaust, after fire, or up through the carburetor, back fire?
Does it 'pop' when you are accelerating or only when you let off the throttle and are decelerating?
If only on decelerating then I would suspect an air leak in the exhaust.
As mentioned, gotta be some kind of timing/ignition problem. Good idea pulling a few things apart, as the nylon quiet cam gear, is known to be a problem over time. If this is warn to the point, the chain starts jumping teeth, could be some walking in your near future.
First find out how much you've got and why- wrong vacuum can, missing limiter bushing etc. IOW- map the advance curve, I think Lar's papers cover this.
From: At my Bar drinking and wrenching in Lafayette Colorado
Originally Posted by moonsteel
How exactly do I cure "too much vacuum advance"?
You set up your timing and vacuum advance as outlined in my papers. E-mail me if you need copies of the timing paper and vacuum advance paper - specific instructions for altering the length of the timing curves are in the papers.
Popping in the exhaust is primarily the sign of a lean condition - you're going to want to richen it up in addition to fixing the timing.
Small update (spent my weekend under a buddies 69 Mustang).
My vac canister says "B9", which (if I'm not mistaken) is a VC1605.
I made only one change this weekend, turned on Decel Fuel Cut. Backfire is much less violent and happens less often as well.
Next step, recheck my timing.