[Excaliber]

Could be timing all right. Is it "advancing" beyond base timing setting? You can check that with a light.

Another common item is the accelerator pump shot in the carb. Take the air cleaner off and look down the throat of the beast. When you "snap open" the throttle with your hand you should see a "squirt" of fuel being shot into the venturis on both "primary" sides. If not, the engine goes instantly "lean" and backfires or pops.

A couple of quick checks for you, I'm sure there is more to do, maybe you'll get "lucky"!

Ernie