I have a 2001 Yamaha f15 tiller. All maintenance is current. Rebuilt the carb about 30 hours back. I have a new problem that I don't know where to begin. I searched the internet for answers, but nothing concrete. The motor surges about 200 RPM's at WOT, but performs perfect the rest of the throttle range. I use ethanol free gas treated with SeaFoam. Gas line is new, filter is new, carb is spotless. Any ideas?