A fuel pump has a diaphragm, driven by pulses from the crankcase or a cam, and a pair of one-way valves that let fuel in one side and out the other. In this video I test the low pressure pump from a Mercury EFI outboard with a little pressure and vacuum on each port, which shows whether the diaphragm is torn or the one-way valves leak, then replace its valves with a repair kit. I also show the metal reed valves in a Yamaha two-stroke fuel pump.
A faulty pump can starve the engine, or a torn diaphragm on a two stroke can push fuel into the crankcase and make it run rich. I rebuild one with new check valves, diaphragm and gaskets.
The barb the fuel line pushes onto can snap off an outboard's plastic fuel pump. I grind the break smooth, tap a thread and fit a brass fitting, then explain the risks.
When fuel pressure drops too low, an EFI outboard won't run properly. I test pressure with a gauge, then strip the vapour separator tank to check its strainers, high pressure pump and regulator.
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