Ok. I'm sure this question is vague and that if i searched for 10 hours i might find an answer, but I liek the direct approach, and my first HOUR of searching and reading yeilded little answers and added more confusion. I just bought a used but great condition 2007 250 XCF-W with a Big Bore Kit. I think the bike sat for the last year mostly. ANyway, initial ride of 5 mins was okay. It had a little bit of gas that the seller left in it (not sure how old the gas was). I put bike in storage for 2 weeks till i could ride again. Then i put new fuel in it (93) and rode it for 15 mins just fine, but suddenly lost all power under any throttle and ALOT of backfiring. Bike idles fine but can't twist throttle without stalling, cutting off, and rapid backfiring. Again, idles like a dream, but acts like it isn't getting enough fuel during throttle. Actually, come to think of it... it even revs fine when in neutral (but will likely still backfire), but it won't run under load (me riding it). Maybe that is helpful to you gurus. VERY bad backfiring, ZERO power. I did see a flame on a backfire. I ride in the same altitude/climate that the seller is in, so no changes there. Drained the new gas a put in new gas but no fix there. Changed oil and filter just before this happened also (just fyi). Otherwise, I'm a total newb to KTM and to newer dirtbikes in general. My last dirtbike was an XR200 back in the mid 80s. I'm not too afraid to work on mechanical things, and I have all of the proper drawings, docs and build manuals for this model KTM. However, considering that the bike ran fine for hte first ride and then for the first 15 mins of the 2nd ride, I am inclined to think that the problem isn't severe. Probably something small like a fuel clog or something. ANyway, thought I would ask the pros..... Thanx for reading... Thanx again for answering.