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The list is long. (if you have an exhaust leak, you should be able to hear it)

Does the truck idle on petrol and gas?

pertonix
Extended rotar button
Air leak
Plugs
leads
Tappets

Have you got yourself an extended rotor button?

Backfire will be air leak in the exhaust system or your gas is running lean
on the condenser there should be a screw with a spring on it , back it out two flats of the hex.
this will allow more gas...

Start with checking the timing and ignition system.
Cheers, the ferret.