Extreme surging at idle.

[Bart1015]

Hello all,

My friend's M151a2 has recently started surging and I cannot seem to be able to figure it out. I hope someone here can shed some light on the issue.

The truck has been running great. But a few weeks ago, the truck began having issues with holding an idle. It would idle for a short time and then it would surge and die. It was drivable by using the hand throttle to keep the idle above normal. Then, after a few times on the road the truck started surging consistently and it died if I don't hold the hand throttle open a bit. The surging is described as: The idle will drop very low and then it will rev well above a normal idle speed. This happens over and over and will not stay at one speed. It almost runs as if one spark plug was disconnected. If you rev and hold the engine at a high idle it will run fine.

The truck received a partial restoration in 2019, 600 miles ago.

When the body was restored, the engine received the following:
- New Coil, installed electronic ignition, cleaned up the distributor, this is a prestolite unit.
- New complete HD head. I did lap the valves.
- Rick rebuild the carb, this carb has the vent return line to the tank.
- Resealed engine with gasket kit.


These are the things I have done to try to figure out the issue:
- Checked distributor to ensure it was tight. Can't check timing because I can't get it to idle at a normal speed.
- Used a propane torch to check for intake leaks. Found none.
- Cleaned and checked the plugs for function. The plugs appeared ok.
- Checked the ignition wires for resistance. No issues found.
- Swapped out the USGI coil for another known good USGI Coil. No change.
- Made sure all vacuums lines are in good shape without cracks breaks.
- Installed a new inline fuel filter (aftermarket, installed between the pump and carb).

The idle mixture screw is not working. But, this may be because I can't get it to idle at a normal speed.

I do have another rebuilt carb I could try but I would rather figure this out instead of just swapping out parts.


I am kind of at a loss and any help would be greatly appreciated.

Brett

[rickf]

Brett, It sounds like you have covered all the normal bases. It sounds like a vacuum leak to me and I am not big on the propane trick especially with that fan moving as much air around as it does. Try using carb cleaner and spraying it around the intake manifold while running, especially be sure to get the intake ports on the bottom. If that does not work try the other carb and if that solves the problem send me the carb I did for you and I will fix it. Have you checked the fuel pressure? Is this an A2 with the engine driven pump? Could be a bad or dirty valve in the pump and loosing pressure.

[Bart1015]

Thank you Rick.

I will get some carb cleaner this week. I’ll also check the fuel pressure at the carb. It has the A2 fuel setup with the pump at the engine.

I seem to recall installing a NOS fuel pump when I did all the engine work. At least it looks brand new. But NOS is still really old and I am sure the internals are not compatible with today’s fuel. The pressure check seems really valid.

I’ll post an update when I work through the tests/checks.

Brett

[muttguru]

"- Installed a new inline fuel filter (aftermarket, installed between the pump and carb)."

Bart,
try this (yes, Rick....you know what I'm going to say )

Remove the in-line fuel filter and reconnect the fuel-pipe and try running the engine again.

Ken

[rickf]

"- Installed a new inline fuel filter (aftermarket, installed between the pump and carb)."

Bart,
try this (yes, Rick....you know what I'm going to say )

Remove the in-line fuel filter and reconnect the fuel-pipe and try running the engine again.

Ken

And if it works then you will know either the filter did it's job and it is clogged or the fuel pump is on it's way out and is having trouble pushing fuel through the filter which should be no problem at all.

Yep Ken, and you knew I was going to reply with that!

Sorry but I have been running inline filters on all sorts of vehicles going all the way back to the 30's and no issues.

[m3a1]

I just skimmed over what has been discussed thus far. Have you checked the condition of the fuel pickup sock in the tank?

[Hambone]

Sorry but I have been running inline filters on all sorts of vehicles going all the way back to the 30's and no issues.
[/quote]


If you're that old, you don't need to be out there working on that shed.

[Bart1015]

Sorry for the delay in my update. I ended up working out of town with little notice.

This is what I have done since my last post:

- Checked for vacuum leaks. Found none.

- Checked fuel pressure at the carb (used a T with gauge at the factory filter). The pressure was just under 3lbs with the factory pump. No change in how it ran.

- Used an external fuel tank with an electric fuel pump right to the carb. The electric fuel pump is regulated at 3lbs. No change in how it ran.

- Swapped out the carb. It started and ran for a short time. The engine stopped and I was unable to get it started back up. When I took the air pipe off, it contained a lot of fuel. It looks like the carb was dumping fuel into the carb.

Next:

I looked at the diaphragm in the original carb and found three pinholes around the metal disc shaped cup.

I took the diaphragm from the second carb and put it in the original one.

The truck started right up and idled ok. When giving it gas, it dies. It's dying right at the common "flat spot" point, but I cannot get it to stay running past that point.

- Note, the fuel air mixture screw does work now, it did not before the diaphragm swap.

Then....

After about 5 minutes of idling, the engine stopped as if it was turned off and it would not start back up.

I checked for spark, it had none.

Looked at the coil and for it was leaking (leaking is a recent issue as I previously checked this coil). I replaced the coil with another known good unit. It started right up.

Now it idles again but it still dies when giving it any gas above an idle.


Other:

I do believe I have the diaphragm installed correctly. The spring is on the outside, being pushed in by the diaphragm cover. I wonder if the debris from the diaphragm holes are stuck in a jet?

I am thinking of buying a few soft carb kits if I can find any that works with todays gas.

Any ideas or thoughts?

[rickf]

The only soft kits I know of are the military kits and they predate modern fuel. They will work fine for a fairly long time as long as you keep the gas fresh. If you get a case of phase separation or a bad load of gas with extra alcohol in it then you will have problems. As much as Ken hates inline fuel filters this is why I push for their use, to catch the crap from the tank and lines and pump that will go right through that original filter. Been there and done that. I would just open the carb up and with the pump out (yes you have it together right) Blow out the passages with carb cleaner. Spray into the main jet and this will push out anything in the accelerator pump passage. Take out the two jets in the bowl and make sure they are clean. If this is the carb I built you are much better off with the pump diaphragm I used then an older one.