Suzuki engines are very good, I have a 50cc oil injected Suzuki clone engine in my Hyosung scooter, and it RIPS, 40 mph with my 215 lbs...the smaller older bikes were restricted by the factory, Honda too, to meet moped laws in the '70's check your exhaust for being clogged, the exhaust port too, old oil did not burn clean and left lots of carbon behind, check air filter too.