I am totally new to the TT forum so help me if you can. I just purchased a used 2003 WR450 with 110 miles and need some guidance and updates. The original owner told me it was completely stock. He had an aftermarket pipe on it for a while (brand unknown) and removed it before he sold it to me. It has stock exhaust now except I installed a Yamaha GYT-R exhaust insert to replace the stock insert. The problem I am having is a persistent hesitation in the throttle, sometimes coming from an idle up to 1/4 throttle and sometimes from 1/4 up to 1/2 throttle. At full speed the bike rips, although I am going to remove the 150 stock MJ and install the extra 155 that came with the bike to compensate the increased flow in the exhaust. I have read about the airbox mods, etc and plan to do them but I don't think this is my problem from idle to 1/2 throttle. Another thing I noticed was after revved it sometimes will idle high before it returns to normal idle, similar to opening the choke. I asked the original owner about it and he told me it had not been ridden since last year and it ran fine then. :confused:If anyone has any suggestions for a starting point on jetting, etc I would greatly appreciate it.