When I was young I had a friend that had one of these. I remember it going faster than 25 MPH. More like 50 MPH on a level road.
This all depends on the year. Most were sold between 1971 and 1978. If you have one that is this old it is very possible it is due an overhaul. The rings could be worn. There are seals inside the crank case that can leak which will cause a loss of power. There could be a problem with the rotary valve. It could also be a plugged mufler. Oil and gunk can accumulate in the muffler. Remove the exhaust pipe and clean it out with solvent.
Even if you have a newer model you may still have some of these same problems.
