Hello after having many two strokes i decided to purchased a 1999 yz 400f after riding my brothers 426.... before i bought the bike i was told it needed a rebuild but still runs, i started it up and it idles fine and starts pretty much first kick, it has good compression and all, but when i give it throttle it backfires and splatters, i changed the oil and filter, and when i took it up the street it runs fine but just doesnt have the power or balls and is slow as and when i back of it backfires i cleaned the carby and airfilter checked exhaust no leaks & good packing. could it be the timing or valves?? or am i just in need of a Rebuild! any help is much appreciated Thanks Adam. Here is a youtube video of the bike running.