M38A1 Fuel Pump

rick25 · #1 18-11-12, 03:05

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 · #2 18-11-12, 03:21

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 · #3 18-11-12, 03:28

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 · #4 18-11-12, 03:43

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 · #5 18-11-12, 04:37

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 · #6 18-11-12, 16:54

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.

rick25 · #7 19-11-12, 00:03

So, I opened up the fuel side of the pump and moved the 3 rubber inlet valves with the flat end of a ball point pen and they did in fact appear to be stuck.

Not knowing how to disassemble the rest of the pump, I removed the metal valve retainer for the outlet side and *very carefully* coaxed those valves open with the round tip of the ball point pen.

I then re-installed the pump and had fuel pressure almost right away!

Now I just need to fix my vacuum leaks and figure out how to adjust the carb.

Thanks Rob!

rob love · #8 19-11-12, 04:00

Too easy eh? Glad it worked out for you, and cheaply too.

Casey B · #9 19-11-12, 05:18

Hi rick I am glad and releaved that the carb worked! Good news for you then.
There aren't many adjustments to the carb other than the one idle screw on the side...and the timing. set the timing...it should be alright and now adjust that screw so it idles at about 800 to a 1000 rpm...then adjust as it heats up...it should run a little faster when it heats up! so then turn it down and use choke and hand throttle to start when colder outside.
Great to hear you got it going. I am still interested in those wires Rick. Just back from a FTX and heading out in 2 weeks for another with army...winter FTX in interior mountains. I'll keep in touch...Casey