Your 99 Yamaha big bear 350 starts fine runs a few minutes then backfires and dies could this be a bent valve and out of time i have cleaned carb and cleaned tank and put new gas?

The symptoms you're describing—starting fine, running for a few minutes, then backfiring and dying—could indicate several issues, including a bent valve or timing problems. However, it could also be related to fuel delivery issues, spark plug problems, or an ignition system failure. Given that you've already cleaned the carburetor and replaced the gas, it would be prudent to check the valve timing and inspect the valve clearance to ensure they are within specifications. If those components are in good condition, consider examining the ignition system for faults.