98' YZ400F Backfire Question

I have just recently got my bike together and fired it for the first time in over a year. I bought it broke. I put a WR head and cams in place of the YZ head that was trashed. I also changed the timing to match the original YZ timing. It starts up easily but with the choke on it revs too high. I tried to turn the throttle screw but all the way in and all the way out was no different. I never did get it to idle without me holding the throttle to keep it running. When I gave it gas it was very responsive but after letting off the throttle it would back fire a couple of times. Nothing violent but I know that it shouldn't happen that much. Any suggestions of where to start would be helpful. I have checked the exhaust for leaks and found none. Something else that may be helpful is that the guy that owned it before me ran it on race gas. I'm going to just run it on 93 octane. THANKS AGAIN. :D

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.

One or two pops out the exhaust is normal. Excessive backfiring can be the result of an air leak at the midpipe or header/head joint, or lean idle mixture. The way you describe yours, I wouldn't concern myself with it if the bike runs alright and you can't find an exhaust leak.

Pump premium is the right fuel to use.

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