M38 Fuel Pump Advice Needed

[dpcd67]

Ok, another chapter in this M38 restoration. Got the original AC fuel pump professionally rebuilt. It does produce 5 pounds of vacuum when you push up on the arm. But installed in the jeep, it produces 1/2 pound of suck and zero blow. Technical terms of course.
The arm has about 1/2 inch of play in it.
The cam is good on the pump lobe; I used a dial indicator mounted on the front engine mount and it shows .260 inch of camming action.
I am using the 9/16ths iron spacer.
So, I am thinking that the pump needs to be closer to the cam so the arm can get more movement.
Thinner spacer. Which I can make. Or mill down the original one.
What do you think?

[Joe Gopan]

The WO-800361 0.566" thick spacer is the only one used on the M-38 and M-38A1 Fuel pumps The Civilian Vacuum Booster Pump used on CJ-3B and CJ-5 uses the thinner approx 1/8" Spacer. Something is wrong inside your pump. There were two different Arms made for the M-38 and M-38A1 Pumps, and they have to natch the rod on the upper diaphragm. Going to guess that your pump was repaired using parts that are not made for your particular pump. M-38 ans M-38A1 shared the whole assembly. There were at least 3 different part number pumps that got used on the M-38 and M-38A1 Pumps, The support units that repaired them did not differentiate the different models and installed them on either Jeep. Get out the ORD 9 for G-740 and G-758 and you will see.
Your rebuilder may have installed parts that do did not match your core.
Do not monkey with the spacers, you will damage your cam, or Rocker Arm.

[dpcd67]

It's the same arm that came on it and it came on the jeep but I know nothing about these. Clearly the arm has too much slack in it before the diaphragm moves.
And the arm was like that before rebuild. Slack in it. Maybe that is why it sat in the weeds for 30 years before I got it....
So until I get this sorted out I can either put an electric one in line, or get a repro civilian one; they make ones with dual chambers I think.

[Joe Gopan]

The Civilian version requires the thinner WO-647689 Spacer.

[dpcd67]

Yes it does.

[Wolfman]

You have been checking the vacuum on the intake side of the pump.
What about the pressure on the outlet side ??
Once the pump builds up pressure, there will not be any suction because the pressure holds the diaphragm down and the pump stops pumping.
Possibly a blockage on the outlet side ??
I would consider a check valve in backward but you say it pumps by hand, off the engine.
How are you checking diaphragm movement with the pump assembled, mounted on the engine ??

[dpcd67]

No, I said that I checked the outlet side and it produced zero pressure when mounted on the jeep. When pushed by hand, after taking up the slack in the arm, it produces over 5 pounds.
Checking diaphragm movement? it moves only when you take up the slack in the arm, which is about 1/2 inch, then it will make good pressure. By hand.
Mounted on the engine, I can't check diaphragm movement. But per the vacuum gauge it pulls 1/2 pound and pushes zero on the outlet side.
Mounted, the arm is not actuating the diaphragm, I think. I think the arm has too much slack in it, but as I have never had one of these fuel pumps before I do not know how they are supposed to be.
If you know, please tell me how much free play the arm is supposed to have. I was told by another guy that the arms are only supposed to have very little free play.

[dpcd67]

Talked to Then and Now where the kit came from; they said the slack in the arm is not correct and to send it to them; they have parts and know what to do.
He said he sees that a lot; incorrect parts on them.

[Joe Gopan]

To those unfamiliar with M-38 and M-38A1 Fuel Pumps, they are a double action with vacuum booster on the bottom. There are a couple three variations that all appear similar, but had changes in the internals. As the years went by, the earlier M-38 variation would find its way into the M-38A1 stockpile during overhaul at the Army support level which was OK.
Civilian life, not all who repaired them were familiar with the internal differences.

[JMZ421]

Then and Now rebuilt my pump and they did a good job. I remember there was no play at all in the pump arm as I tried to actuate the arm and it was tough to do. No slack. I did use that thick spacer and made sure the arm was lubed where it contacts the cam. I also used those longs bolts with lock washers as the one bolt became loose. I believe Then and Now sent me two paper gaskets in the return box. John

[dpcd67]

Ok , good; that verifies it; no slack in the arm. Yes, you have to use the 9/16ths spacer, and the two long bolts go on the M38; makes installing it far easier than on WW2 jeeps.
Mine is on the way to Then and Now.