Abstract: A changeable Venturi carburetor includes an auxiliary fuel which has a vacuum valve and a temperature delay vacuum valve. The vacuum valve has a third air nozzle connecting with a second air nozzle in the temperature delay vacuum which further has a first air nozzle communicating with the main throttle of the carburetor. Through the temperature delay vacuum valve, air pressure at the main throttle will be transmitted to the vacuum valve which then controls auxiliary fuel supply to increase engine output power. As a result, engine cold start may be done easily and smoothly, and warm up done in a short period of time. It can also increase engine power by providing additional fuel to the engine during sudden acceleration or high situation.