The power system is incorporated into the carburetor to provide a richer mixture for maximum power and high speed operation. The extra fuel for power is supplied by a vacuum controlled power piston which automatically operates the power bypass jet in accordance with throttle opening. Intake manifold vacuum is maintained above the vacuum piston through a vacuum channel which leads to the mounting flange of the carburetor. During partial throttle operation, the vacuum above the vacuum piston is sufficient to overrule the compression spring and hold the piston in the UP position. When the throttle valves are opened to a point where the manifold vacuum drops to approximately four to five inches Hg, the compression spring then moves the piston DOWN to open the bypass jet and meter additional fuel into the main metering system.