M38 fording system vacuum leak?

JeepdaddyRC · Post by **JeepdaddyRC** » Fri Feb 10, 2023 8:50 pm

Post by **wesk** » Fri Feb 10, 2023 10:40 pm

Jeep daddy,

Did you remove your carb and inspect the visible intake areas for anything left behind?

The front and left front views only yield half of your vacuum plumbing. A full left side view is a must as illustrated below.

Note how visible the heat riser is. Have you checked it's operation per the manual?

JeepdaddyRC · Post by **JeepdaddyRC** » Sat Feb 11, 2023 5:04 pm

The heat riser was rebuilt and tested with a heat gun before the manifold was re-installed. The weight and bimetal spring are working.

Using a branch T fitting at the inlet to the carb, the fuel pressure was 4.1 PSI at 1800 rpm.

Per Wes's previous post:
TM 9-804 does not offer a pressure target in its troubleshooting section but on page 110 (Fuel System Specs) says 4 to 5 1/2 PSI at 1800 RPM at 16" above the pump's outlet.
TM 9-8012 on page 78/79 par 79b.(2) says 4 1/2 to 5 PSI at cranking RPM. In the same manual on page 151 (Fuel System Specs) it states 4 to 5 1/4 PSI at 1800 RPM.

I have assumed a vacuum leak since it ran at ScoutPilot's shop. My next step is to remove the carb and look for foreign objects.

An interesting side note. This is the second YS carb I've found with no check ball or weight - but it ran and idled. Mystery continues.

RonD2 · Post by **RonD2** » Sat Feb 11, 2023 7:34 pm

Hi JeepDaddy,
Half of fuel pump test is pressure --- the other half is volume.

I came across this old thread that might interest you.
Read it all, but see what he discovers on page 2.
http://willysmjeeps.com/v2/modules.php? ... e&start=15

I assume you checked your cast iron intake manifold carefully for cracks, even tiny.

Good luck!

JeepdaddyRC · Post by **JeepdaddyRC** » Tue Feb 14, 2023 3:31 pm

Update. I've assumed a vacuum leak and have been searching for it.
However, the carb just flooded twice. Fuel overfilled the bowl and came out the balance vent fitting.
Something prevented needle from seating? Could not find anything keeping it open. But there is fine rust in the float bowl.
While waiting for carb rebuild, fuel tank was cleaned by radiator shop and new stainless fuel filter installed. My goal was to provide clean, filtered fuel to freshly rebuilt carb. Well, that didn't work as planned.
Perhaps this stirred up some rust/crud that is causing the needle not to seat causing a rich condition and no idle. Does this logic make sense?
Just removed the fuel lines. Blew them out with carb cleaner and compressed air. May consider the NOS fuel filter since the stainless mesh is not doing the job.

RonD2 · Post by **RonD2** » Tue Feb 14, 2023 4:31 pm

Makes sense to me.
Also might have crud in the fuel pump?

Did you also test pump volume?

Did you rinse the tank with gas after the radiator shop cleaned it?

Stainless mesh filter may not have had a chance to do its job if the crud was already past it in the lines and pump?

I also use the stainless mesh filter.
I have no personal experience but heard that NOS filters (ceramic? fiber?) can be fragile and disintegrate easily from age.

JeepdaddyRC · Post by **JeepdaddyRC** » Tue Feb 14, 2023 4:54 pm

Have not measured fuel flow yet, only pressure.
Ever have something prevent the needle from seating?

RonD2 · Post by **RonD2** » Tue Feb 14, 2023 5:03 pm

Post by **wesk** » Tue Feb 14, 2023 11:16 pm

Yes I have had many stuck float needle issues in the shop. Some were damaged seats, some rubber tips partially dissolved and sticky, some with bent or otherwise damaged levers.

It is usually best to open her up and examine what you have with a completely open mind. Compare what you see with your manuals and if still unable to resolve the issue then come here and ask for ideas and help.

It's also a good idea not to leave any question unanswered. When a member suggests several sources for problems or asks several question always try to clear the backlog and answer or respond to all before jumping into the next game.

I'll assume you haven't answered Ron's fuel flow question because you haven't had her running at idle long enough.

We are waiting for a good full left side engine view.

4x4M38 · Post by **4x4M38** » Wed Feb 15, 2023 7:51 am

Ethanol gas will degrade if sitting long enough and gum up your carb, including the needle and seat. Had one so stuck it didn’t move at all.

And yes, particles in your gas can cause/contribute to the needle not seating if stuck in that degraded gas crap. And if particles are in your bowl they can be anywhere in the carb, especially in places with much smaller holes/orifices.

JeepdaddyRC · Post by **JeepdaddyRC** » Wed Feb 15, 2023 4:22 pm

Wes, here are the engine left side photos requested. Black hose from the intake manifold port is for my vacuum gauge. Timing light adapter on #1 plug wire. Balance vent tube from carb air horn to crossover pipe temporarily removed during troubleshooting
Have not measured fuel flow because it will not idle.

JeepdaddyRC · Post by **JeepdaddyRC** » Sat Feb 18, 2023 3:30 pm

Update:
Removed fuel lines and cleaned. Tested fuel at carb inlet. Fuel clean.
Restarted Jeep. Same symptoms, hard to start, only starts with almost full throttle, will not idle. Runs better with a little choke
Again checked for vacuum leaks at all areas with carb cleaner, no change in rpms (however this was not done at idle, because Jeep will not idle).
Removed and disassembled carb again. Confirmed no foreign objects, blew out passages with carb cleaner, confirmed idle passages clear.
Reinstalled carb. Started Jeep. No joy. Same symptoms and same problems.
Sprayed carb cleaner around manifolds, carb base and vacuum fittings. No change in rpms.
Mystery continues. Possible vacuum leak since carb idled in Scoutpilot's shop. But cant locate it.

JeepdaddyRC · Post by **JeepdaddyRC** » Sat Feb 18, 2023 9:48 pm

Pulled off manifolds to confirm no mice or foreign objects. Manifold gasket looked fully sealed. Any techniques to look for hairline cracks? Nothing visible.
I did use cone washers (taper towards nut) on manifold studs 3 and 5. Studs are new.

Post by **wesk** » Sun Feb 19, 2023 2:12 am

This is where you borrow a good carb and install it. If problem goes away send your carb back for warranty work. If problem persists you have now completely removed the carb from your culprit list.

Dye penetrant is a inexpensive method of searching for hairline cracks. Magnetic particle is another.

JeepdaddyRC · Post by **JeepdaddyRC** » Sun Feb 19, 2023 1:44 pm

I wish I had a spare carb.
Reluctant to ask... anyone have a spare I could use for a short time?
It could solve this mystery. Happy to rent it if you prefer.
I will cover shipping both ways.
I am honest and promise to take good care of it.
Only need it for a few days.
Located in Madison, WI