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.