I just bought a 2001 yz250f a couple of weeks ago with the original tires still in nearly new condition. It started the first or second kick, by just kicking it like a two stroke, when I test road it. When I took it on an actual trail ride I ended up turning the idle down because I thought it was idling too high as compared to the KTM I had just come off from. After I did that I could not get the bike to start. Needless to say I was pissed and I didn't put 2 and 2 together!! I made a trip to the local yamaha service counter and they told me to turn the idle screw in to where the bike will be idling around 2,000rpm and it will probably start. Then keep turning the idle down to the point where it is almost hard starting again. I tried it and it starts first or second kick hot or cold now. Hope this helps. I will be checking the valves too.