fuel pump losing prime ?

[Tom Jamboretz]

I have a M151 A2 that loses it's prime if it sits for and hour. I have to pour gas in the carb intake. It has done this for a long time. I've put a new fuel pump on and it still acts the same. Does anyone have a suggestion???????? TJ

[artificer]

I've put a new fuel pump on and it still acts the same.

This is a very expensive way of doing things. Also good reason to diagnose properly & not just replace good parts with other good parts.
Testing/checking the original fuel pump would have ruled it out as the problem & even though you have a new one you should check it. Vacuum/pressure gauge from HF about $15 is a good investment.

Any kind of fuel pump relies on pressure differentials to operate.
If you don't have these differentials you have a problem like yours.
Unless the fuel pump is an in tank type, you will no doubt find there is an air leak caused by loose union/s or leaking gasket/s between the fuel tank & fuel pump.

This will not necessarily show as a fuel leak @ any potential leak point & usually doesn't.
It will show if you low pressurize the fuel tank then check right to the fuel pump for fuel leaks or dampness. Then repair each one.

Leaks after the fuel pump will dangerously show, if the engine is running [potential engine bay fire] & will not cause what you call loss of prime.

[Rickf]

Are you using the choke and does the choke close all the way? Without a manual accelerator pump to inject gas before you start you have to use the choke to start a lot of the time even when hot. Also make sure the intake hose from the carb to the air cleaner is left on, if not you lose the last shot of gas that is injected into the carb throat as it shuts down. There are one way valves in the fuel pump to prevent gas from running back to the tank but that would not cause the problem you are having since the carb bowl holds more than enough gas to start the engine and keep it running long enough for the pump to pull gas from the tank. The pump draining back will not drain the carb bowl.

[Tom Jamboretz]

Gentlemen, I thank you for the interesting comments. That gives me much to think about and diagnosing to do. I'll be getting on it soon. Thanks, TJ