Testing Pump Pressure


The pump pressure is determined by the degree of spring compression during the intake stroke of the pump. The spring tension is so calibrated that it allows,the fuel to enter the carburetor only as long as the float needle valve is open. When the buoyancy of the float forces the float needle valve to close, pressure builds up in the fuel line and pump fuel chamber shortening the pump stroke. In normal operation, the diaphragm stroke amounts to only a few tenths of a millimeter.

The lower assembly is vented through two orifices in the casting. Also, should the fuel leak into this part of the pump, it can drain through the venting holes.


The pump pressure should be 0.20 to 0.24 atmospheres (2.9 - 3.5 psi) with the float needle valve closed and engine running at 1,000 - 3,000 rpm. Minimum fuel delivery should be 30 liters (7.9 US gal.) per hour, which equals 500 cc (16.9 us f1. oz.) per minute, at 4,500 rpm.

The simplest way to check the fuel pump pressure is with the aid of a pressure gauge inserted into the fuel line between the pump and carburetor by means of a T-joint. A fuel shut-off valve is incorporated in the fuel line behind the pressure gauge.

Essential for proper pump pressure is correct spring tension and faultless condition of the diaphragm and control valves.

Excessive pump pressure causes carburetor flooding and, in almost all cases, leads to dilution of oil. Insufficient pump pressure causes lean combustion mixture and, thus, a rough running engine with misfiring at high rpm and decreased power output.
Last modified: Tue, 21 Dec 2004


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