Reply To: fuel pump pressure

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Jim,
Fuel delivery systems must satisfy two conditions: Zero demand before start-up and maximum demand on a long mountain grade. Gravity systems; vacuum tank or Ford Model A, can deliver fuel at nearly fixed pressure during all conditions. Mechanical pumps deliver fuel proportional to engine speed; very low during cranking and increasing with engine speed.

Electric pumps must meet both flow conditions with fixed output, often by using a regulator for pressure control. Too much pressure will flood if the float valve does not shut tight, but too little will starve the engine under heavy load.

The electric pump my ’29 PA uses has a separate switch under the dash – not direct to the ignition. Separate switches allow the engine to start with the fuel already in the float chamber. Then this switch is used to provide flow when running, or run it just a few seconds on a hot day to prevent vapor lock before cranking.

Setup of the carburetor will also have a large influence on the pressure needed or tolerated. If you want to dig deeper, send me an email. Herb