what normally causes backfireing out exhaust, is it from bad spark plugs or a carb needing adjustment

I would say you are jeted too rich, too much fuel geting wasted in the pipe causing back fire.

good call Bertram

bertram i have a 03 prox 800 w/ slp single pipe air horn kit and flow rites
w/factory 450 main jets w/the needle set at#3. do you think i should change anything to stop it from backfireing or wait till i get to run it and see if it gets better

Your carb settings look good. tell me when it back fired, if the engine was cold and it poped it means it wants more choke, but if engine is a operating temp and popes through pipe check your carbs, could be a stuck choke.

unburnt fuel in the pipe causes exaust backfires..... if you hold a steady rpm and it pops you more then likely have a bad plug! it will happen most often with a cold motor!