M38 mystery

1950 - 1952, M38, questions, discussions, regarding anything related to the M38.
JeepdaddyRC
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Re: M38 mystery

Post by JeepdaddyRC » Mon Jun 05, 2023 9:10 am

Update:
I now have a new head gasket, rebuilt carb, rebuilt fuel/vacuum pump, new brass fittings and new vacuum lines. Timing 7-8 degrees. Have spark and air. New fuel filter. Fuel tank clean, fuel lines clean or replaced. Fuel vent working. New points and condenser.

Still acts like a vacuum leak. Needs almost full throttle to start. Idle vacuum is 18.5, but engine runs better with 1/2 choke and the vacuum increases to 19.5-20. There is a significant hesitation off idle and the engine wants to die unless the choke is half closed.

These symptoms suggest vacuum leak to me. Agree? But unable to locate a leak using propane enrichment and/or and carb cleaner.

Mystery continues. Thoughts welcome.
42 GPW #91366, recovered blue drab registration 20200841, unit markings 104-415-I HQ-21 (Timberwolves)
CJs (x 4)


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dpcd67
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Re: M38 mystery

Post by dpcd67 » Mon Jun 05, 2023 10:09 am

I always blame the Carter vacuum operated carburetors used on Dodges, the M38 and A1. They are evil and designed by satan.
And I am usually right. But check for vacuum leaks anyway; plug all the fording vent lines off, etc, and see if that helps. Use the spray carb cleaner method of finding a vacuum leak.
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Scoutpilot
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Re: M38 mystery

Post by Scoutpilot » Mon Jun 05, 2023 10:42 am

Did you test the underside of the intake manifold?
'70-'71 'A' Batt., 377th FA, 101AB
(Slicks and LOH's)(Col's LOH Pilot)
'71-72 CHARLIE TROOP,
16TH AIR CAV, 1ST AVN BDE (AEROSCOUTS)
LOW LEVEL HELL.
'46 CJ2A
'47 CJ2A
'48 CJ2A
'48 B1PW126
'69 CJ5

JeepdaddyRC
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Re: M38 mystery

Post by JeepdaddyRC » Mon Jun 05, 2023 4:37 pm

Rick, do you mean the underside of the intake manifold to block mating surface? Or, do you mean the joint where 4 bolts join the intake and exhaust manifold by heat riser?
Dpcd67, now that made me laugh. I’m beginning to think you are right and this carb is haunted. Prefer to keep it original but almost ready to break down and try a Carter WO on it, can’t find another known good Carter YS 637S to try out to see if it’s carb or something else!
42 GPW #91366, recovered blue drab registration 20200841, unit markings 104-415-I HQ-21 (Timberwolves)
CJs (x 4)

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dpcd67
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Re: M38 mystery

Post by dpcd67 » Mon Jun 05, 2023 7:05 pm

Right; I keep a known good WO on hand for just such things; those are simpler, have a mechanical accelerator pump and as long as they have clean gas they will work.
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Re: M38 mystery

Post by Scoutpilot » Tue Jun 06, 2023 12:39 am

JeepdaddyRC wrote:
Mon Jun 05, 2023 4:37 pm
Rick, do you mean the underside of the intake manifold to block mating surface? Or, do you mean the joint where 4 bolts join the intake and exhaust manifold by heat riser?
I mean the entire manifold.
'70-'71 'A' Batt., 377th FA, 101AB
(Slicks and LOH's)(Col's LOH Pilot)
'71-72 CHARLIE TROOP,
16TH AIR CAV, 1ST AVN BDE (AEROSCOUTS)
LOW LEVEL HELL.
'46 CJ2A
'47 CJ2A
'48 CJ2A
'48 B1PW126
'69 CJ5

JeepdaddyRC
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Posts: 289
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Re: M38 mystery

Post by JeepdaddyRC » Tue Jun 06, 2023 9:06 am

Just tested for vacuum leaks again. Spraying a heavy stream of carb cleaner with engine at idle (needs 1/2 choke to idle) there was no change in rpms when directing spray at top and bottom of intake manifold to block mating surface, between intake and exhaust manifolds, carb base gasket, carb throttle shaft, carb joints, PCV plumbing, fording valves, valve cover plate, intake manifold vacuum line fittings to fuel/vacuum pump, fittings at wiper T, special .040 restriction fitting for distributor vent line (wipers are capped off), distributor vent fittings (top and bottom), rear air cleaner fittings for vent lines to the fuel tank and master cylinder (MC line capped at MC).

Perhaps this is important additional info: This M38 originally had a vent line with a special pressure fitting originating at the center of the fuel/vacuum pump to a line ending at the transfer case. That 90 degree fitting was replaced by an axle vent when Scoutpilot did the fuel/vacuum pump rebuild. I believe that center opening on the fuel/vacuum pump goes to the crankcase. Could this new axle vent cause my problem?

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42 GPW #91366, recovered blue drab registration 20200841, unit markings 104-415-I HQ-21 (Timberwolves)
CJs (x 4)

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Scoutpilot
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Re: M38 mystery

Post by Scoutpilot » Tue Jun 06, 2023 9:11 am

No. What is the idle RPM? If close to 1000 you may not discern a difference.
'70-'71 'A' Batt., 377th FA, 101AB
(Slicks and LOH's)(Col's LOH Pilot)
'71-72 CHARLIE TROOP,
16TH AIR CAV, 1ST AVN BDE (AEROSCOUTS)
LOW LEVEL HELL.
'46 CJ2A
'47 CJ2A
'48 CJ2A
'48 B1PW126
'69 CJ5

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Scoutpilot
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Posts: 7337
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Re: M38 mystery

Post by Scoutpilot » Tue Jun 06, 2023 9:14 am

PM sent.
'70-'71 'A' Batt., 377th FA, 101AB
(Slicks and LOH's)(Col's LOH Pilot)
'71-72 CHARLIE TROOP,
16TH AIR CAV, 1ST AVN BDE (AEROSCOUTS)
LOW LEVEL HELL.
'46 CJ2A
'47 CJ2A
'48 CJ2A
'48 B1PW126
'69 CJ5

JeepdaddyRC
Sergeant Major of the Gee
Sergeant Major of the Gee
Posts: 289
Joined: Thu Jan 26, 2017 11:32 am
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Re: M38 mystery

Post by JeepdaddyRC » Tue Jun 06, 2023 12:01 pm

Idle rpm 650-700. Thanks for hanging in there with me!

Scoutpilot, I don't see a pm from you. Did you send thru g503?
42 GPW #91366, recovered blue drab registration 20200841, unit markings 104-415-I HQ-21 (Timberwolves)
CJs (x 4)

JeepdaddyRC
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Posts: 289
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Re: M38 mystery

Post by JeepdaddyRC » Thu Jun 15, 2023 11:56 am

Mystery solved - it was a fuel delivery problem. I installed a working carb (civilian 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.

Since I need to return the borrowed WO carb, one option is to purchase a new repo Carter WO from Ron Fitzpatrick. Anyone have experience with those? But this M38 is original and I prefer to keep it historically correct. I prefer a working YS637S. Anyone have a trusted source for one, or one they would be willing to sell?
42 GPW #91366, recovered blue drab registration 20200841, unit markings 104-415-I HQ-21 (Timberwolves)
CJs (x 4)


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