I have a 2000 Yamaha WR400F which I was riding on Friday. I was riding at 300ft elevation, ~28 Deg Celcius and it was raining lightly. The first half of the day the bike seemed to be running well, this was mostly on double track. The second half of the day we were mostly on single track and was around the time I started to notice issues with the bike. At first the bike would stall when I went to take off. Stalling sometimes when I went to give a little gas even with the clutch fully dis-engaged. Eventually it didn't want to idle at all, and was getting to be hard to start. After a while, I noticed that some times while starting the bike, I didn't need to use the decompression lever, I could turn the motor over without using the decompression lever. Today, I've been fooling around with the bike a bit. I can get it started, but can't seen to get it idling. It'll idle for a bit and then die on me. If I give it some gas I can keep it running, but as soon as I stop givving it some throttle it'll stall after 30 or so seconds. Any ideas what the issue may be? I'm going to grab a leak down tester to see if the motor hasn't blown a seal or valve. The bike still seems to run good once moving, but does seem to have more deceleration popping and backfired a few times. I rebuilt the top end of the motor this spring shortly after I bought it. Last Friday was my fourth ride on the bike (each day being about 4-6 hrs). I haven't touched the carb too much, and don't know what the jetting is. But I'm assuming that there's no reason why it would change all of a sudden.