no offense to anyone, but i dont think a lot of people realize the sport maxx is a single sped tranny. it is geared inbetween the regular t maxx tranny for performance.
your little rubber plug on the top of your tranny case is for shift point adjustment, you dont have a 2 speed tranny. the case is used if you decide to switch to a 2 speed. there is no sence in retooling for a new case just for the sport maxx.
you might want to check your idle setting, what might be happening is your still reving a little high on deceleration and the clutches in the single speed are still trying to engage.
check the throttle adjustment knob on the radio and adjust as needed. then check the idle setting on the carb.
you just might have to back down a little.
its a starting point. not to many people i know of out there with the sport maxx. its a new tranny and its problems arnt well known yet if there any.
jim