A friend of mine has a Manco Talon with a Linhai 250cc engine. The tranny is the CVT type. When that machine is placed in reverse the engine cuts on/off to keep the speed down, this really makes backing up a hill difficult. Yesterday when we were riding I almost had to pull him back up the hill with my quad as all the Talon would do is spin/stall/spin/stall.

I have not looked for a sensor, but I am sure there is one on the reverse gear/shifter/tranny that feeds back to the CDI unit.

 

This looks old so you probably dont care anymore but I had this happen on my Manco Talon and just got it working by disonnecting the 'Speed Limiter' module which is mounted by the CDI box. I ran it in reverse it work so I reconnected the limiter and all is still ok. There is also an adjuster on this module that sets the limit, clockwise to increase and CC to decrease the limit.