Hey I have a Yamaha ct2engine on a 1976 100cc frame. It runs like a demon but I have a issue. Everytime I try to shift into third it feels juss like the chain is badly slipping or missing teeth on the gear and then I shift to fourth and its fine but down shifting it gets hung up between 2nd and3rd and I have to shut the bike off and work the shifter to get it unstuck and back to 1st . Any ideas? This thing has me stumped