All bikes are custom built using either a 2 stroke kit or a 4 stroke kit. The 2 stroke kit has a clutch lever and you pedal the bike up to speed with the clutch pulled in and release it to start the motor. The 2 stroke engine requires a gas-oil mix to run. The 4 stroke runs on plain gas. The 4 stroke motor has a rope starter and a centrifugal clutch. You just hit the gas and go. The 2 stroke bikes are cheaper due to the lower cost of the engine.