Abstract: A fuel regulator receives fuel from a fuel pump at essentially constant pressure and reduces the pressure of the fuel to a constant low value in response to a predetermined high to moderate vacuum in an inlet manifold of an internal combustion engine, the vacuum corresponding to low or moderate load. At high load the regulator supplies fuel at a pressure proportional to absolute manifold pressure. At low to moderate loads the fuel at low pressure passes into the float bowl of a carburetor when a float controlled needle valve opens in response to a low level of fuel in the bowl as fuel is burned in the engine. The low pressure does not unseat the needle valve and supply an excess of fuel to the bowl. At high loads, the fuel pressure to the bowl is adequate to supply the increase rate of fuel such loads demand. As a result, the engine operates lean at low to moderate loads and richer at high loads. The regulator has a first diaphragm controlled valve.