A 21.5? What speed control are you using? Is it in no timing 'blinky' mode, or something with boost?

That's likely going to need a huge pinion, especially if blinky, with the smallest spur they make. For reference the 17.5 blinky guys are typically using 30 to 35 tooth pinions with the smallest spur (69-70 tooth or so). A 21.5 likely needs a larger pinion than that. Some speculation on my part though, never tried a 21.5.

Even then I suspect it would be down on power, offroad typically starts with 17.5. 21.5 is only an on road thing, maybe oval, to the best of my knowledge. Depending on the brand of motor you might be able to just swap to a different wind stator, otherwise suggest getting a different motor.