Mechanical Fuel Pump issue?
 

I've been lurking in this area all summer trying to get to the bottom of my engine shutdown issues. The long story short is that the engine will shut down after several minutes of running at half or higher power under a load, but seems to be able to run for extended periods while idling.

Early summer I rebuilt my carb and decided to yank off and look at the mechanical fuel pump at the same time. I found a broken spring in the fuel pump, replaced that during my rebuild with the kit from moyer. I thought this would help resolve my issues, I wrong.

So I decided to get the shutdown warning system from moyer and add a fuel pressure gauge while I was at it, it is all installed as of today, but the warning system hasn't been wired up yet.

Current behavior is that I can pump the fuel pump manually and get 6 psi at the carb. But once started and while running at an idle I am barely above 1 psi, and if I throttle up the pressure drops to barely registering above 0 on the gauge. While running I can manually pump the fuel pump and get it back up to 6 psi, only take 2 or maybe three pumps to get it there. While doing this I can feel the rod from the engine trying to run the pump.

Before I yank off the fuel pump again and take a look to see if I did something wrong, does anyone have any other areas to look into or any thoughts on my issue?

I did a bunch of other unrelated work on the engine back in the spring, I upgraded to electronic ignition, and replaced the plug wires, along with checking that other parts of the electrical ignition system were working.

At 1 psi the engine will mot get enough fuel to make much power at all. You need 2~3 at least. Time to check the pump and check valves. Not a bad job at all.

Also do be sure that the clamps and fittings all the way to the tank are tight so no air leaks. take a good look at the filters too.

You could also go the electric pump which is a good pump but not as good or safe as the mechanical pump IMHO.

Dave Neptune :cool:

Thanks for the reply Dave, I would prefer to stick with the mechanical pump if possible, so I want to get to the bottom of this.

I had on my list to check connections back to the tank so I will do that next. If I have a bad connection would I still be able to manually operate the pump and hit 6psi?

Yes, you could still hit the high pressure but with little volume or flow for higher RPM's and power.

With the bail you have far more travel for pumping whereas the engine only gives a little pulse (travel) far more frequently.

Dave Neptune :cool:

You may have a problem with the pump. Maybe the new spring was installed incorrectly. Or it could be an obstruction in the filters. Did you replace filters when rebuilding the carb? If not, junk might have gotten past the filters into the check valves in the pump when you operated the bail, since the bail will create more suction than the pump. I rebuild and clean my carb every year and replace filters then but haven't touched the mechanical pump in 20 years. I vaguely remember noticing back then that the check valves could be installed wrong. Changing to an electric pump is not necessarily an upgrade, IMO, although they are easier to replace. Once your mechanical pump works well, you can probably forget about it for years, if the fuel and filters stay clean. Use non-ethanol fuel, and especially treat the fuel with MMO, to keep the rubber diaphragm and valves in the pump in good shape.

IMHO, the mechanical pump is a good thing. It's weakness is the strainer basket on the inlet which is useless and prone to leaks. I think Moyer sells a replacement strainer basket. Still useless, but less likely to leak.
I changed to electrical pump and used it for years. Issues with it are 1) that it needs the oil pressure safety switch, and 2) it's electrical. Every time it cycles to pump fuel, a little switch is going OFF/ON. I never trusted it.

There was a dude on here that had removed the basket part of the pump and had the inlet coming straight into the pump diaphragm body itself...I had an extra pump to experiment with, but have always had something more pressing on the list, since like capn ward, I got my mech pump working and have not really had to mess with it for years.

+1 for another guy that prefers the mech pump over an elec. setup.

Putting fears into perspective
 

Driving your family car must be nerve wracking then. When was the last time you saw a car with a mechanical fuel pump?
Full disclosure: it turns out one of my cars has one but it's 53 years old

Which pump is standard equipment on MMI New Block and exchange engines?

Seeing the post Shawn mentioned about a fuel pump with the sediment bowl removed I gave the modification a try to see how easy or difficult it was. With the proper tools (1/8-27 pipe tap and the proper sized drill bit) it was very easy.

Some of us have no choice, my engine never had a mechanical pump.

I have often wondered what the sediment bowl is for, since on the rare times that I open it, there is no sediment in it, nor is there anything caught in the screen above it. When I rebuilt the pump 20 years ago, the bowl did have some sediment. I guess the inline filter just upstream of it catches the sediment first. The Racor water separator is before that. I also have another inline filter between the pump and the carb. Since we can't have clear filters on inboard engines, we can't tell if they are getting clogged, so I replace those filters every year, they're not too expensive. It's as if the sediment bowl is a relic from the days when there were no inline filters, if there ever was such a time. Removing the bowl should be done after closing the fuel valve and running the engine dry, otherwise you are more likely to spill gas. It is tricky to re-install the bowl in a way that doesn't leak. The gasket (which should be fresh) and screen have to fit just right, and it needs to be tightened well. My 1972 Datsun pickup had a mechanical pump; never had a problem with it in 15 years. That was when boats our size were built with gasoline engines. How archaic!
Moyer's use of electrical pumps is plenty of recommendation for me, but I still like my mechanical pump. It requires no electricity, or OPSS. The hand bail will pressurize the fuel line without turning on the engine, so no need to bypass the switch. I have read on this forum that mechanical pumps create more suction, so small air leaks in the line are overcome, and don't put air into the pump to stop it working. (Not that any air leaks in the fuel line are acceptable.) My pump has no problem pulling and pushing gas past a check valve, three filters, and a pressure gauge. It's true that if the diaphragm in the pump breaks, it will leak fuel into the crankcase, which is a real concern. After 5000+ hours, I am still waiting for that to happen. But I expect not much fuel will go into the crankcase, since the engine will not run. Using MMO in non-ethanol fuel postpones that day, IMO.

Neil thanks...those are the details I had just never worked out yet. I would also eliminate the hard copper line and use a soft A-1 fuel line to the secondary filter and then the inline pressure gauge.

Agreed Shawn. If it ever found its way onto my boat the copper line would be a goner.

The only copper line left on mine is the scavenge tube. I see no need for copper fuel lines in this day of high quality "rubber" hoses. I trust hose clamps more than compression fittings. I think copper fuel lines are descended from the era when hoses were actual rubber, which was hard to get then and dried out and cracked over time. Possibly during WW2, when Universal was producing the Utility 4 for lifeboats, the military preferred copper lines because hoses could puncture and burn from enemy fire, but now that isn't a problem, except maybe in Ukraine, bless them. Besides, if you have a fire anywhere near a carburetor, you are screwed anyway.

Progress update: going is slow, I've got two toddlers pulling me in other directions. Luckily they still like to get down on the boat and help dad hold a flashlight and grab tools. It adds time, but gets them involved.

I ran a new fuel line from the pump directly into a separate external tank, still had the same issue with real low fuel pressure. So the issue is clearly the pump. I yanked it off and brought it home to look at it on my work bench. No obvious problems like incorrectly installed check valves, no obvious leaks. The top doesn't appear warped, and the sediment bowl is serviceable. I also closely inspected the rocker arm where it contacts the pusher rod from the engine, it also appeared to be serviceable with no noticeable wear.

I plan to remount it to the engine and see if fuel pressure improves. I am not hopeful since I didn't find anything wrong. Any last ideas on items to check before throwing up my hands and changing to the electric pump? Is there a way to check that the pusher arm driving the pump from the engine is traveling the proper distance?

Thanks for all the advice and input so far!

While you have it off, why not just rebuild the pump? I rebuilt mine about 10-12 years ago and it provides 3-4 PSI no problem. :confused:

If you switch to electric, you will need to add the low oil pressure switch, one more complication.

I did rebuild it back at the start of the summer, same time I rebuilt the carb. I found a broken spring at that time and thought it would help resolve my low fuel pressure issue.

When I took it apart just now I was double checking my rebuild, ensuring the check valves were oriented correctly and all gaskets were installed properly.

When you did your rebuild, did you replace the check valves?
I had a fuel issue some years ago where the check valves were not functioning properly -- one was stuck and not opening properly -- a poke with a small screwdriver unstuck the check valve and did the trick. Once the check valves were functioning properly, my fuel issue was fixed. (Thank you, ethanol...). What about the diaphragm in the pump - did you replace that also? When you tighten the two halves of the pump, make certain you've tightened them evening to ensure there are no leaks. (Obvious stuff, I know...)

https://moyermarine.com/product/mmi-...fpmp_11-1_357/

You can buy a whole new pump.

Or another rebuild kit and give it another go:
https://moyermarine.com/product/mech...t-fpmp_13_320/

You don't have to go electric. If my engine had a way to use a mechanical fuel pump I would probably use one.

Resolution, and some humble pie
 

Last update -
Posting this so others can learn from my mistakes and ensure they absolutely check everything.

The long story short is I did not completely remove the old gasket from where it was stuck to the side of the block. I didn't have a great view and was reinstalling it by feel the first time around and the old gasket was holding the pump about 1/16" farther from the block than it should have. This reduced the work the pusher rod could do to pump fuel.

In hindsight, I suspected that something felt off when I would play with the bail while the engine was running and could only barely feel the pusher rod at almost the end of the range of travel.

So with the old gasket fully removed, I removed the carb and got a few other things out of the way to give myself better access to that side of the engine, it now pumps like a champ. Giving me between 3-5 psi of fuel pressure! And now while playing with the bail while she is running I can feel the pusher rod strongly doing work through much of the range of travel.

Bottom line, all the details are important. I spent much time with the pump itself, and I neglected part of the reinstall that ultimately caused problems down the road. I used my phone to take video from different angles during the second install to get a better view which lead me to pull off the carb to get better access and fully ensure that part was done correctly.

So now I move on to the next issue...

Good find! That would puzzle a lot of us and a mechanic would probably just go buy a whole new engine.