M38 fording system vacuum leak?

JeepdaddyRC · Post by **JeepdaddyRC** » Thu Mar 02, 2023 9:21 am

RonD2, there are photos of my vacuum gauge connection to the intake manifold in my Feb 16 post. I followed the instructions on page 80 of TM 9-8012: "disconnect fuel and vacuum pump vacuum line at elbow located on exhaust manifold, and install vacuum gage adapter in elbow (fig. 22)."

My carb rebuild performed differently on different engines. I do not have Scoutpilot's vacuum of 23, but my vacuum of 19.5 is not bad.

I am out of ideas. I still suspect a vacuum leak. Since the carb likes choke, I have too much air or not enough fuel. Since I can't find an air leak, my next step is to send the fuel/vacuum pump to Scoutpilot. I have found him very helpful and a pleasure to work with.

JeepdaddyRC · Post by **JeepdaddyRC** » Sun Jun 04, 2023 8:29 pm

Update:
I now have a new head gasket, a rebuilt carb and a rebuilt fuel/vacuum pump (thank you ScoutPilot). New brass fittings and new vacuum lines. Confirmed fuel tank vent working. Timing 7-8 degrees. Have spark and air. New fuel filter. Fuel tank clean, fuel lines clean or replaced. New points and condenser. Gaps set to specs. Plugs look tan.

Still acts like a vacuum leak. Needs almost full throttle to start. Vacuum reading at 650-700 rpm is 18.5 But engine runs much better with 1/2 choke and vacuum increases to 19.5-20. Significant hesitation off idle and wants to die without 1/2 choke.

Can't locate a leak using propane enrichment and/or and carb cleaner. However, when I spray carb cleaner on the two fittings at the rear of the air cleaner, the rpms pick up and the engine smoothes out. This confuses me. I thought those air cleaner connections for the master cylinder and fuel tank were vent lines only and did not lead to vacuum leaks. The connections are tight.

This has been a challenge indeed, but perhaps this new info will help us solve the mystery. Thoughts welcome.

Post by **wesk** » Sun Jun 04, 2023 9:38 pm

Try to remember that air cleaner feeds your carb which is sucking on it. Anthing flammable you introduce up stream (AWAY FROM SUCTION) will increase the RPM as it finds it's way to the Suction source called your carb.

JeepdaddyRC · Post by **JeepdaddyRC** » Mon Jun 05, 2023 10:53 am

Wes, you have a way of making things make perfect sense.
I could not have come this far without you and this great forum.
Would you agree the symptoms suggest a vacuum leak?

Post by **wesk** » Mon Jun 05, 2023 11:32 am

Maybe but still not sure. Could be vacuum or it could be mixture. Once you eliminate all vacuum leak options with a propane type check. The next check is mixture adjustment check along with a recheck of fuel pump pressure. Also if you have kitted the carb up make sure you have the right metering rod installed. A lot of kits come with both for the m38 & M38A1.

Here's a dual carb kit.

I wouldn't waste a lot of time worrying about a vac leak at the fuel pump. Has very little to no influence on manifold vacuum. The fuel/vac pump vac side only does one job. It increases vacuum suction to help the wipers perform adequately. If you insist on following a troubleshooting path towards the vac side of your fuel pump then you only need to perform one step. Remove the line from the intake to the vac side inlet of the pump and plug the hole in the intake.

JeepdaddyRC · Post by **JeepdaddyRC** » Tue Jun 06, 2023 7:19 am

The carb has been apart many times, including a rebuild by Scoutpilot. The carb kit is from MWM. I confirmed accelerator pump and metering rod spring sizes are correct and in proper locations. The diaphragms are good and it is the correct M38 metering rod size. Scoutpilot drilled out the new low speed jet tube orifice to correct size with #70 drill bit and believed the orifice on the 3/16 carb vent line fitting on top of carb was too small. However carb to crossover vent is clear and no difference in performance with carb vent line on or off.

Regarding a vacuum leak at the fuel pump side, when I attach the vacuum gauge it essentially takes the pump and associated vacuum lines to wipers and distributor out of the equation. The vacuum gauge is plugged into the intake manifold port that normally leads down to the pump.

Carb base gasket and intake manifold to block mating surface would be my first suspect areas. Neither indicated air leakage to disrupt the mixture. Throttle shaft on carb would be my other suspect area, no sign of leakage there. Beginning to think the carb is haunted.

The only way this Jeep will start is to pull the hand throttle way out while cranking. This is different than my other experiences and seems inconsistent with a vacuum leak and more like a flooded engine. This one has me stumped indeed.

JeepdaddyRC · Post by **JeepdaddyRC** » Thu Jun 15, 2023 1:44 pm

Update: Mystery solved - it was a fuel delivery problem.

I installed a working carb (Carter WO). The M38 now starts easily, idles smoothly, has no hesitation on acceleration and the phut-phut from the tailpipe is gone. It is an unbelievable difference, it's like a new Jeep, sounds great and purrs like never before. ! asked my daughter to confirm I wasn't dreaming.

But now I need to return the borrowed WO carb. One option is to purchase a new repo Carter WO from Ron Fitzpatrick. If anyone has experience with those, I would appreciate a review.

But this M38 is mostly original and I prefer to keep it historically correct. Therefore if anyone has a working YS637S they would like to sell, please let me know.

Thank you to Wes, Rond2, and all that helped me with this mystery.

RonD2 · Post by **RonD2** » Thu Jun 15, 2023 2:25 pm

Glad you found the problem!

I'm surprised to hear that a YS-637S that ScoutPilot rebuilt has a problem.
Before buying another (big bucks), have you contacted him to see if he'll fix it under warranty?
Just a thought.

4x4M38 · Post by **4x4M38** » Thu Jun 15, 2023 9:10 pm

Using the WO civvy carb eliminates a lot of stuff besides the YS637.

The carb may still not be your problem.

I’d send it to Scoutpilot and tell him about swapping to the WO, and ask him to stick it on his test stand.

JeepdaddyRC · Post by **JeepdaddyRC** » Fri Jun 16, 2023 11:13 am

Brian, other than eliminating the tube to crossover pipe and using a mechanical accelerator pump and metering rod, what else does the WO eliminate? Why do you think the carb might not be the problem? All info is helpful from more experienced M38 owners. I do realize the YS vacuum operated systems are designed for fording, but since the WO was used on the CJ3A it is was a step in my trouble shooting. The starting, idling and performance improvements were unexpected and amazing. It sounded like a completely different engine with fuel delivered by the WO.