Outboard fuel pump operation. 

   The diaphragm-type fuel pump used on most outboards is operated by crankcase pressure. This type of fuel pump cannot create sufficient vacuum to draw fuel from the fuel tank during cranking. They are not very effective at self-priming. Air is purged from the fuel line and fuel is transferred to the carburetor, for starting purposes, by hand-operating the hand primer, installed in the fuel line. (often called a "primer bulb") Small air leaks in the fuel line, between the pump and the tank, will dramatically reduce fuel pump efficiency and fuel flow volume. 

ob engine block w/diaphragm fuel pump system


Pressure pulses created by movement of the pistons, inside the crankcase,  reaches the fuel pump through a passageway between the crankcase and the pump. The pulse passageway could be cast into the block or a hose run between the engine block and the fuel pump body.

Upward piston motion (compression stroke) creates  low pressure in the crankcase, and pulls on the pump diaphragm. This low pressure opens the inlet check valve in the pump, drawing fuel from the line into the pump. At the same time, the low pressure draws the air-fuel mixture from the carburetor into the crankcase.

Downward piston motion creates a high pressure, in the crankcase, and pushes on the pump diaphragm. This pressure closes the inlet check valve and opens the outlet check valve, forcing the fuel into the carburetor and drawing the air-fuel mixture from the crankcase into the cylinder for combustion. The diagrams shows the operational sequence of a typical outboard fuel pump.

ob diaphragm fuel pump close-up


Diaphragm fuel pumps are extremely reliable and simple in design. Diaphragm failures are the most common problem. Since the diaphragm pump can leak fuel directly into the crankcase, it is important to know that an engine that is flooding and/or running too rich can be caused by a broken diaphragm. The other critical component of the fuel pump is the two opposite facing check-valves. The use of dirty or improper fuel-oil mixtures can cause check valve problems.

It is also important to know that some diaphragm pumps are not serviceable and should not be disassembled. Check your service manual for specifics.

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