I just swapped an Honda XR for a 2003 TTR 225 (plus some money!). The Yamaha is in very good shape - adult owned, the guy seemed honest, the bike doesn't look abused in any way etc. ...but it didn't run right. He told me: - just had it serviced (not sure what "just" means but let's assume recent just before he parked it last year) I know where he had it serviced - I take all my bikes there and they're a good shop. I trust them. - I saw for sure he replaced the battery (it was dead - the bike was definitely not used much). - had old gas in it, and he drained it an put new gas in it - he never had problems with it - 100% stock (except for a K&N airfilter) I am still 100% convinced he is honest so I don't doubt what he told me. I made the exchange and figured I could fix the bike without too much trouble. I was a little over confident on that one. It starts right up. It warms up okay then it won't take gas with or without the choke. Choke makes very little difference once it's running. As soon as I give it gas - it's as if it is super lean and it dies out. I was able on one occasion to rev it up high (don't know how) slipped the clutch, rode it 1/4 mile wide open - ran fair, fought it as I turned around, revved it up, wide open on the return trip then I gave up. I was afraid of being stranded and pushing it back. So my conclusion was the main jet seemed reasonable. Testing it after I got it home (at night), the header starts to glow cherry red almost immediately. To attempt to fix it I: - took out the K&N airfilter - no problems, seemed clean and normal. I peeked in the airbox - no rat's nest or anything unusual. all symptoms the same w/o an airfilter - new Yuasa battery shows 12.2 volts hooked up with the ignition on - i replaced the sparkplug (old sparkplug looked better than I expected. chocolate brown); no difference with new plug - i removed the carb. soaked jets in carb cleaner, blew out passages etc. Carb didn't look that bad but i cleaned it meticulously anyway. no difference - I removed the plug over the pilot screw. It seemed like the factory setting was 1/2 turn out ? (doesn't seem right) I turned it 2-4 turns out - absolutely no affect on the symptoms no matter how many turns out. Because there is no special intake modification (no holes in airbox, snorkel in place) and factory exhaust, I gotta think the jets are about right. (I live in Richmond, VA - not an altitude problem either) - Then I thought airleaks. I didn't use a scientific method but with the bike running, i squirted some soapy water on the intake manifold. Not a conclusive test but it didn't seem to make any difference. Now I'm frustrated because I can't change the symptoms. Could it be timing? Weak spark? Any ideas from you seasoned veterans?