View Full Version : Fuel Pump Question

10-07-2012, 12:39 PM
The other day while winterizing the boat motors i noticed a fuel leak on the Yamaha 25hp 4 stroke. The leak was happening on the fuel pump between the outer cover and the main body that are seperated by a paper thin rubber diaphragm. Anyway, got the leak fixed with some Permatex gasket sealant. While i had it apart i noticed that the outer medal cover had a couple of recessed areas that the rubber diaphragm sits on top of. My question is what's the purpose of the diaphragm and how does it work?

10-07-2012, 03:15 PM
In the fuel pump, 1 side of the gasket is for fuel. The other side is for crankcase pressure. The crank case pressure goes in and out like a person breathing. It moved the gasket ( diaphram) back and forth. The other side( with the gas) pulls gas from the fuel tank and forces it to the carb. It will not let gas go back to the tank if the small flapper valves are good. They are in another layer of the pump amd there are usually 2 of them.
If the pump did not work, Look for leaks. Squease th primmer bulb. It will show were the leaks are.
1- connecter to motor
2- fuel pump cover.
3- Internal pump gasket- will not prime up to hard bulb- will show leak into water
4-carb overflow- needle and seat stuck open
5- no gas to carb- needle stuck close
6- if no gas to carb other than when squeezing the primer bulb- possible leak in fuel pump pressure hose- not all motors have this hose. some get pressure from the back side where the pump mounts to the block.

10-07-2012, 10:24 PM
The fuel pump works fine...just wondered about the diaphragm. Thanks for the great explaination.