If you have a pin hole leak in the carb diaphragm, it will suck air and not go very fast. The bike is old, it has never been changed, take my word on this. Order a new one, pop off the carb top under plastic cap-under the seat. 3 screws or 4 (don't remember), and lid lifts off. Beware not to lose spring. Put needle jet and plastic piece back in, in correct order with spring in same position it was in when you took it apart. Really very easy to do. You may need to smear a little grease on the lip of the new diaphragm to make it stay down inside the recessed area before placing spring with top-cap back on, but once you push lip in groove of top of carb you will know if you need the grease or not. Also, adjust the valves. They tend to loosen up over time/miles. Loose valves = a slow bike that sucks gas. Good luck.