what all of these guys are saying is true. first never rev it in the air, you take a big risk in snapping the pins in the trans. but i would go with leaning you engine out a lil first to get it to rev a lil higher. i work in a r/c shop and see this all the time. and 8 times out of 10 it can be fixed with just a lil tuning to the engine. if at that point it still will not work that try the screw adj. i have seen it where the spring in the 2 speed clutch get alil mess up and week and then they also wont shift! but just tune it to start!