Abstract: The present invention relates to a system for preheating gas prior to entry into the carburetor and further preheating the air-fuel mixture passing from the carburetor into the intake manifold of the engine. In particular, the present invention entails a heat exchanger plate assembly disposed between the engine carburetor and the engine with openings provided therein for allowing the air-fuel mixture to pass therethrough. Additionally, the heat exchanger plate assembly includes a first compartment operatively connected to the engine cooling system and a second adjacent compartment operatively connected to the fuel supply line. The fuel is preheated by the exchange of heat between the first and second compartments, and further since the coolant fluid is circulated through the first compartment and the same is adjacent openings allowing the air-fuel mixture to pass from the carburetor to the engine, it follows that the air-fuel mixture itself is also heated by the heat exchanger plate assembly.