I own 3 rm 125 motors, the 134 has the same powerband as the 125 with nice gains while the 144 is more torquey and less fun to ride...I love all of them, just need more clutch on the 125 motor...

Nice. Apparently 134 is the best size for 125s and reports are that it runs better than a 144.

Yea, i was told 134 was best for mx. 144 would probably be better for trails. I can’t praise Tom over at tmr enough. He’s one of the few that will take the time to make sure your getting what you really want out of your motor.