Race gas is a bad idea, it runs too hot and can burn vital engine parts. My 99 YZ idles really fast with the choke on and barely puts with it off.. you might want to check the operation of your fuel screw, it should make some difference. I'd pull the throttle cable plate (the plastic cover on the right sideof the carb) and ensure that the screw is manipulating the throttle assembly, maye check the tension of the throttle cable? The backfire could be jetting? does it run rough or miss at steady throttle? or only pop on decel, then you chop the throttle closed? If it plain old will not run with out throttle manipluation you should (if not already done)disassemble the carb and clean it, then reassemble with proper jetting, the sticky thread at the top of this forum can help, so can your local Yamaha
dealer, they should be able to asses what jet sized you need for elevation. I dont know how a WR head would change the operation of a YZ, but I'd start simple.