Clark and others, please help. I have posted this on the YZ board and gotten some insight, but cannot remedy the problem. I recently bought a 98 yz4 and have ridden it about 10 times. Love the bike, but every time I ride it, after about 1 hour in the saddle, the bike refuses to idle and backfires like mad. Starts fine w/o choking, but won't idle. The idle screw is all the way in. Any ideas of how I can increase it? I am just above sea-level and the pipe is a FMF Power Core 4 (stock head pipe.) Several other knowledgeable YZ riders say it sounds like there is an air leak. The hot start button is definitely not out, and the hot start hose is hooked up. I have tried altering the air screw anywhere from 1 to 3 turns out. I don't know anything about carbs, so I couldn't tell you the jetting setup. The bike starts fine cold & hot, runs great - holds consistant speeds well, and accelerates really smoothly (makes me not want to dick with the carb.) The bike idles fine when cold, but stalls when hot. If I open the idles adjust screw at all - forget it - no idle ever. Thanks guys.