Are you starting from a dead stop? If not I'm not sure why you would be losing power. If it feels like it's slipping while riding or accelerating you can try a few things. First make sure the cable is adjusted right, this can be checked by looking at the clutch arm, the clutch arm should be parallel with the engine or pushed outwards a little over parallel with the engine. If that is set and it's still slipping you can adjust the flower nut. Take off the large cover that covers the clutch and bevel gears. In the bigger gear is the clutch and there is a large round plate held on by a nut shaped like a flower. (Hence the name flower nut) there is also a screw holding the flower nut in place. Undo that screw holding the flower nut, pull in the clutch lever (locking it helps if you have a locking lever but if not just tie the lever down) then push the clutch plate in towards the engine and spin the nut in about a half a turn, put the screw back in place and the engine cover back on. Try it again if it still slips try turning the flower nut in more. It's a tedious process.
