M38 mystery
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- Sergeant Major of the Gee
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Re: M38 mystery
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.
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)
CJs (x 4)
- dpcd67
- G-General
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Re: M38 mystery
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.
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.
U. S. Army 28 years.
Armor Branch
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- Scoutpilot
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Re: M38 mystery
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
(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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- Sergeant Major of the Gee
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Re: M38 mystery
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!
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)
CJs (x 4)
- dpcd67
- G-General
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Re: M38 mystery
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.
U. S. Army 28 years.
Armor Branch
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Re: M38 mystery
I mean the entire manifold.JeepdaddyRC wrote: ↑Mon Jun 05, 2023 4:37 pmRick, 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?
'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
(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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- Sergeant Major of the Gee
- Posts: 289
- Joined: Thu Jan 26, 2017 11:32 am
- Location:
Re: M38 mystery
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?
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?
42 GPW #91366, recovered blue drab registration 20200841, unit markings 104-415-I HQ-21 (Timberwolves)
CJs (x 4)
CJs (x 4)
- Scoutpilot
- G-Lieutenant General
- Posts: 7337
- Joined: Sun May 28, 2006 1:22 pm
- Location: Asheboro, NC
Re: M38 mystery
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
(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
- Scoutpilot
- G-Lieutenant General
- Posts: 7337
- Joined: Sun May 28, 2006 1:22 pm
- Location: Asheboro, NC
Re: M38 mystery
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
(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
-
- Sergeant Major of the Gee
- Posts: 289
- Joined: Thu Jan 26, 2017 11:32 am
- Location:
Re: M38 mystery
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?
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)
CJs (x 4)
-
- Sergeant Major of the Gee
- Posts: 289
- Joined: Thu Jan 26, 2017 11:32 am
- Location:
Re: M38 mystery
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?
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)
CJs (x 4)
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