Actually there are two very common things that were done to the free spirit to restrict them. The first thing you should do is take off the carb and look at the intake, there may be a plastic reducer sleeve that make the intake only 10mm, you can pop that right out with a screwdriver or drill. If it does not have this resrictor, it may have the horrible one, take the intake off the cylinder and look at the intake port on the cylinder, if you see a cirular metal sleeve, then it have a restricted 1hp cylinder, get a different stock cylinder and you should be going 28+ mph in no time. Usually it is the first and easier of the two restrictions. let me know how this helps.