One way to test is to adjust the linkage so that it is ALWAYS in forward and won't shift into reverse. If the problem goes away, you may have your answer. This issue is relatively often found on Baja Motorsports products (when it is occurring).
Otherwise, clean the carb anyways. There is an excellent guide here:
http://www.battlescooter.com/1.html
The fuel cleaner may work to a degree but is no match for a good carb cleaning.