M38A1 Fuel Pump

[rick25]

I was able to start my M38A1 today - the first time its been run in 26 years!

The carb has been rebuilt (thanks Casey) and the engine will run and idle great but only when I pour fuel into the bowl through the vent line as there is no fuel supply making it to the carb.

I've plumbed all new fuel lines and the tank is clean so I decided to check the fuel pump which has been sitting dry for years.

I disconnected the fuel pump "inlet" hose and connected a vacuum gauge to find that I'm not even drawing 1" of mercury. Is there some way I need to prime the pump or are the diaphragms shot and I require a rebuild kit?

Thanks,

Rick

[chris vickery]

If my A1 sits for a long time I lose prime as well and it takes forever. I suspect too tha I have an old faulty pump. Looking into a freshly rebuilt unit.
I know old diaphrams have a tendency to dry rot and crack with age, especially if they have been dry.

[rick25]

After filling the fuel bowl multiple times through the vent, I suspect it ran for a total of about 3 or 4 minutes and no hint of fuel through the line.

[rob love]

There are six little chack valves in the fuel pump. If any one of them gets plugged open, or dried out, it will not draw fuel. I have found that by getting some fuel into a dried out pump, they will usually work after that.

[rick25]

Hi Rob,

I've removed the fuel pump and have attached a short line to the inlet side to act as a resevoir for fuel.

When I actuate the pump arm, the fuel goes inside the pump and does not exit from the outlet, instead, it shoots back out the inlet side under pressure.

[rob love]

If you have removed the pump, then it's a quick job to open it up and clean inside. Undo the screws on the bottom half and open it up. Sounds like you have maybe an open valve on the intake side, and a stuck valves on the output.

There are no gaskets involved with opening it up other than the diaphragm. Once open, you will see the check valves...they are held in with metal keepers. Each valve is spring loaded and points up or down, depending on it's job. Just poke the seats with a non sharp item, and make sure the little spring forces the seat back.

The fuel which dried up in the pump over all those years will usually form a white powder. Often, there will be other crap stuck in the fuel pump: contaminants from the tank and the lines. Clean out the pump, and perhaps put an inline filter in the short rubber flexline before the pump if contaminants were found.