I got my Radial 621p Last month and have in the 8ight(no mugen jking) its a nice mil just check the needle settings when you get it. My lsn was way off so make sure of all the needle settings before starting. I guess they don't set them at the factory.
How much did you pay and where did you get it from? I run a jp-3 pipe with mine. The pinch on this motor is real tite and the OPS guy told me not to really run full throttle until 3/4 gallon is through the motor. I have also heard to replace the conrod after 1 gallon. My friend OPS which is the same conrod rip out after 1/2 gallon. He was doing high speed runs on the road(not so smart). I have about 1 gallon through mine and dissasembled the motor and had no play in the conrod so I decided not to chane it. Maybe this helps or you already knew, but I love the motor.