I have had my 1954 M37 for a few years now in Orlando FL and I tried to live with what appeared to be vapor lock. I've started down the path of being able to kill the vapor lock (Symptom) and drive the truck for a 100 miles without issue. Someone before me installed a 'jerry' rigged electric fuel pump and removed mechanical pump. I want the truck to be as it should be so, I bought a mechanical pump from John at MWM and then found myself short on time and expertise.
IN order to get the truck running like it should, I've acquired the assistance of a gentleman who's worked on jeeps and others MV's for ~40 years. We're both stumped because now, the mechanical pump is in and we're getting high fuel pressure. The Carb has been rebuilt and adjusted (like 4 times now) and it doesn't seem to be the issue. In fact, when the electric pump was in place, it seems someone disabled the carb float. After running the truck for a bit of time, and then disconnecting the fuel line, there's a ton of pressure. I would like to see if anyone has an idea why I'm getting high pressure with a new Mechanical fuel pump and we're missing some adjustment of some kind. I don't see anything about this in the military manual nor in other places in this blog.
I'm not sure I understood what you said regarding the carb float. Did you reinstall it when you switched to the mechanical pump? If the carb float is installed and functioning properly then you have a faulty mechanical fuel pump. There's a one way check valve in the fuel pump outlet that won't (or shouldn't) open if the carb bowl is full and the needle valve is on its seat. If that valve is not working (or missing) in the pump then it will continue to pump fuel to the carb whether it needs it or not, building more and more pressure. That condition can do a lot of damage to an engine.
I've never done it, but I think you should be able to check the pump outlet valve on the vehicle. Disconnect the outlet pipe from the pump to the carb and put your finger over it. Then pump the priming lever on the pump body. If the valve is working correctly the pump handle should move freely and not blow your finger off the pump outlet. If the valve is not working then pushing the priming lever should blow your finger of the pump outlet..........or not move at all with your finger over the outlet. Again, I've never done it, so if that's not the case then hopefully someone will chime in. But understand, the mechanical fuel pump does have internal valving to control the fuel coming into and out of the pump. Basically, if the carb doesn't need the fuel, the pump shouldn't be pumping it. Yours wouldn't be the first new or rebuilt fuel pump to not work properly.
Bob, First of all, thank you for your quick reply. Yes, the carb has been rebuilt since installing the new pump. All things seems to lead back to the mechanical pump not functioning as it should. That makes a lot of sense, I will give it a try tonight. We'll see what we can figure out at this point.
Yes, the pressure is high enough to unseat the needle. It might be the float being to heavy. That's where we're at at this point. Pulling off the carb again. Getting there and not letting it beat me!
Yes, like Just Me, what is the current fuel pressure reading?
If you have a problem with the pump that you bought from MWM,
why not call John and discuss the issue with him? I would be
prepared to give him your fuel pressure readings.
Unless you KNOW the actual pressure on the outlet side of the new pump,
you're shooting in the dark. A "ton of pressure" tells you nothing.
If the new pump isn't giving proper pressure, CAll JOHN.
Hi, this issue came up a few years ago during my Willys MB fuel pump rebuild. I was told not to use the the replacement fuel pump spring to diaphragm only use the original. Due to differences in machining the new replacement spring had a tenancy to create "excessive" fuel pressure and flood the vehicle. Just an idea since its a new replacement pump maybe that's an approach to investigate. After talking to MWM maybe try to get a hold of a factory spring?
This reply is long over due. I started this thread with the fact that I was having what I perceived to be fuel issues. The electric fuel pump that was 'Jerry' Rigged into the truck seemed to suck and in fact it did. What resolved it in the end, I bought a new mechanical fuel pump from MWM and installed it. I also removed the mechanical pump completely and have never needed it since, even with the heat of Florida summers. The other issue I did find which seems to be fuel related but was not, the voltage regulator was cooking my coils regularly. This caused what seemed to be vapor lock but in fact was a bad voltage regulator. New fuel pump is still running strong, new voltage regulator is doing great and I have not replaced the coil since the regulator was replaced. I hope this helps someone. Weeks.
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