Ok, well I have a 1999 YZ250. Its been used for racing it had a Pro Circuit Platinum Pipe on it and was jetted for it. I got it the guy had taken the pipe off and left me with the stock but didn't rejet. It fouled plugs left and right. I got the stock ones and it still fouls plugs left and right after putting them in. Any ideas on how to get it running right?? Someone said maybe there i gunky oil in the bottom of the tank? But can that be the reason?