I have a motobecane 50L modified into a 50V. Stock variator, switched the gearing to standard 50V, 15 SHA with a 62 jet, stock pipe, and mild porting on the stock cylinder. On the kickstand, the thing revs up like crazy, but once you get on it, it bogs out and hits 25 tops on a city block straight away. It's not 4stroking, just horribly slow. The timing is on, the spark is strong, the engine variates on the stand, but barely when riding.
Any ideas? Could a small air leak on the intake be the culprit?