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[16ACE27]

Originally Posted by Jimbo56

Back in the old days, a carbureted automobile had a diaphragm in the gas feed called a vacuum pump. It’s function was to hold gas up in the line to the carb so it didn’t take forever to start the car. You could tell when that little rubber diaphragm went bad by pulling the throttle link and being able to see gas go into the carb.

Does anyone know if an Onan 4000 has such a critter? I think not. Ace, Duck, Bob?

Pulling the throttle and seeing gas go in the carb is caused by the accelerator pump - a simple diaphragm pump integrated in the carb to richen the mixture on acceleration by squirting fluid from the bowl directly down the carburetor throat.

A vacuum pump is used on small engines and is also a simple diaphragm pump external to the carb to move gas to the carb from a fuel tank that is too low to gravity feed the carb. It is powered by the vacuum pulses in the engine and is simply a fuel pump.

Automobile engines before fuel injection mostly used a mechanical diaphragm fuel pump attached to the engine to suck gas from the tank and push it to the carburetor. They were normally attached to the engine itself and mechanically driven by a rod or lever that was driven from the camshaft.

All diaphragm pumps have inherent check valves that prevent fuel backflow so the fuel line would not empty unless the check valves leaked or the engine set so long that evaporation played a role is removing the standing fuel.

Electric fuel pumps are generally centrifugal pumps so when they are not running it is easy for fuel to drain back through them. Modern cars may have two fuel pumps and inline check valves to maintain fuel in the lines when not running.

I suspect the Onan does not have any inline check valves and the small bowl of the carburetor is easily emptied by driving and evaporation when the generator is not being used: hence the need to almost always prime the generator before staring. I would also suspect that if you dug into the circuitry and operation of an AGS that it does have a priming function that runs prior to simply starting the generator. I don't know that for sure but if I was designing one for an onboard generator that's the way I would do it.