If it is backfiring even during idle, there are a couple of possible issues. Check the obvious and least expensive first. Make sure you have correct OEM spark plugs, check ignition wires. Then check the ignition coil and trigger module, there is a procedure for doing that in the Cummins Onan manual. A manual can be obtained from http://www.advanced-power-technologies.com/ or local Onan RV dealer. This will give you the ignition coil resistance and the ignition module resistance. Also has anyone messed around with the carb, or has it sat for long periods, too lean a fuel mixture can cause backfiring. Good Luck ;)