50hp Merc - Hissing noise coming from fuel pump after priming bulb

[simplystressed]

I have a carb'd 50hp merc and notice the fuel bulb loses prime fairly quickly after priming it up....whether starting it or not. After priming up I notice I can hear a hissing noise from the fuel pump on the motor where the in/out hoses go. Can't see any gas leaking and the gasket seems tight it almost sounds like its inside the housing.

Is this normal to hear? I'm going to try soap/water around there to see if there is a leak but wondering if its upstream or something in the fuel pump itself causing it to lose prime. Please let me know what you think.

Thanks!

 

[hotrod53]

Re: 50hp Merc - Hissing noise coming from fuel pump after priming bulb

I was told that if you over pressurize it, this can happen. Do you have Tillotson carbs with the blue inlet pieces? These are known to warp and leak, ESPECIALLY if you have removed them. Mine leaked there, they are still available and only $5 each. They have 1 bolt in the middle and the tightened pressure causes them to lose their dome shape. Even a new gasket or flat sanding the plastic will fix it, guess how I know? But you're not seeing fuel leaking though, right?

Also keep in mind that you have a brass needle sittting in a brass seat being closed off by a brass float sitting in fuel, it wouldn't take much pressure to force past it. Maybe youre forcing fuel past the needle into the throat of the carb and therefore in the motor? Fuel lines and pump balls can leak or suck air too.

 

[CharlieB]

Re: 50hp Merc - Hissing noise coming from fuel pump after priming bulb

It is also possible that the fuel pump diaphragm has a pin-hole in it and you are hearing fuel squirting into the vacuum side, leaking into the crankcase and causing that cyl to run rich.

Disconnect the fuel pump but leave all fuel hoses connected, pump the primer and watch for any fuel leakage from the backside.