ORIGINAL: quietnas1
With the engine off make sure the carb itself is returning quickly to a closed position. Even though the servo is moving fast doesn't mean the carb is closing fast. If it is closing slowly I would take the carb apart myself. Make sure everything is clean and the correct ”O”” rings were used.
These traxxas carbs are pretty simple.. But if it was just taken apart, perhaps the carb isnt seated all the way down into the crankcase. You may want to put some RTV silicone around that o-ring just to eliminate a chance for a leak. Otherwise, I suggest inspecting the throttle return spring. A weak servo will move the throttle slowly, causing throttle response to be slow, and a slow return to idle. I had it happen in my Jato - needed a stronger servo and the phenomenon went away. The return spring is pretty stiff, and the stock traxxas throttle servos arent strong enough.
Just my thoughts.