This is a stock 49cc 4-stroke carb that has been bored and reworked. Has a larger .478" bore (stock bore is .425"). Circuitry has been reworked. Throttle shaft and butterfly have been modified. Expect quicker revs and more power in mid range to top end. Uses stock return spring so no throttle linkage is required. Our suppliers are always out of stock on these carbs. Send us your carb to be bored and reworked. We charge $45 plus the return shipping.