440roadrunner

First, define what you mean by backfireing

Does it pop back through the carb, or

crackle and pop out the exhaust when you "let off" the throttle?

I'm assuming it pops back

Under what conditions does it do this?

alll the time?

when cruising?

when under power?

when under heavy power?

at high rpm, medium, all ranges?

A rythmatic popping, like a dead cylinder, can be caused by a flat cam load on an exhaust valve, and of course sticking valves, broken springs, burned valves, etc

(Run a compression check)

Are you ABSOLUTELY sure that

the plug wires are in the right order? (Very easy to switch 7-8-

The plug wires are good? Check them with an ohmeter

Is the distributer cap new, clean, and dry? Pull the cap and look for "carbon tracks" between two connections

Can you get a smooth idle? Do the idle mixture screws have a noticeable effect?

Try this: Get a small screwdriver, or a test light. Ground the screwdriver with a wire/ clip lead to the block. Pull the boots loose off the distributor cap. With the engine at the smoothest idle you can get, short out the spark at each cap tower one at a time. Watch a tach if possible. EACH CYLINDER should drop the idle about the same amount of RPM, and the engine should "sound" about the same on each. CONGRATULATIONS!! You've just performed a "cylinder balance test" with no special equipment.

If any one or two cylinders doesn't "drop" the same amount, it indicates a problem in that cylinder

If two cylinders that are physically adjacent on the block are weak, you might have a head gasket leak between those two cylinders

How did the plugs look, any evidence of something like antifreeze leaking into the cylinder?

440roadrunner

A rythmic pop sounds more like a weak cylinder. Read my post, take a compression test, and try the cylinder balance test.

You may have more than one problem, namely something in the idle circuit of the carb. Do the idle screws have good effect?

NorAspen

Does it sound a bit like this?