My 450 atv backfires when i let off the gas - 2005 Honda CRF 450 R

either the float height is wrong, the main jet needle needs lifted or there's an air leak somewhere.

backfiring is usually a sign of an air leak in the intake manifold

Remove and clean the plug, sounds like it is fouled.

Remove the seat, and take the cover off the air filter container. Place your hand over the end to aid in fuel flow to the carburator. You might not be getting enough fuel which is causing it to backfire and not start completely.
Attempt to start the ATV once you get fuel flowing you should be all set. Clean that filter as well.

Check for bad gas, water in gas, blown head gasket. or carb need complete rebuild.

the fuse could be blown its by the battery under the seat also check air filter possibly dirty carb

Check the MAF-MAP SENCER or it's a throttle position sencer is bad check both by replacing them and see if it works or the last result is idle control valve

carb needs overhaul or high speed jets need ajustn--hav a pro du this cause if u lean it out too much-u can fry the engine--clymer makes a mechanics manual for motorcycles and atv

well since i owned the 2007 i know this answer i wouldnt run anything under 91 octain and the better the gas is the better it runs if you run cheap gas it could backfire and pop alot more thats what mine did
hope this helps

Sounds like the timing chain broke.