[quolloyd]

My 1994 Chevy C/K 3500 Dually backfires when the engine is warm. It doesn't happen with a cold engine.

 

[kendallt]

Different things cause different backfires. If popping through carb, it's firing early or VERY late. Normally timing chain or distributor turned, or plug leads in wrong order
If popping through exhaust, it normally means a air leak.

Check to make sure there are no leaks in the exhaust system, air getting in will ignite any unburned fuel, with modern cars the leak would be ahead of the catalytic converter(s)
Make sure you aren't running rich, a bad 02 sensor will cause a rich condition.

 

[octaneman]

Hi quolloyd

I take it that your truck is a 7.4 L 454. From what you are describing the problem maybe a blocked catalytic converter, low fuel pressure or a vacuum leak.
Are you getting any codes ?