My 78 Garelli gulp matic 2speed it seems, is starting to have some problem. When the speedo hits 25mph it sounds like it's winding out. There's a higher pitch rattle, that sounds like it's turning to fast for it to keep up with itself. Now, less than usual will it catch and accelerate smoothly up to 30-32mph. Could this be as simple as tightening my chain? there's a little lag, other than that, I'm stumped. Any suggestions?