not sure what the stock gearing is on those, but going up 5 teeth on a pinion gear is a HUGE increase in the ratio and will put a much larger load on the motor. Normally, with a otherwise stock model, you can only go up 2-3 teeth on the pinion gear...any higher and you exceed the capability of the motor and thus it will generate a lot more heat.

do you know what size the stock spur and pinion were?