Abstract: A fuel supply system for internal combustion engines in which two different fuels are used, one to start the engine, to operate it at idling speed and to supplement the other fuel during acceleration, and the second to operate the engine at its normal working speed. The first fuel may be propane for starting, idling and for an acceleration supplement. The second fuel for normal working speed, or normal driving speed may be gasoline. A first vacuum controlled valve assembly delivers the first fuel such as propane from a pressurized supply tank to the carburetor below the conventional butterfly valve of the throttle control to start the engine and operate it at idle speed. A second vacuum controlled valve assembly delivers the first fuel, such as propane, to the carburetor above the butterfly valve only when the accelerator opens the throttle and its butterfly valve to initiate acceleration, and only during acceleration.