willysmjeeps.com

The profile location is incorrect but I haven't updated it. Should be Edinburg. Jeep is stored in Corpus. I'm heading up there tonight so should have some updates.

Ok, in reference to previous replies, upon preliminary examination, the spark plug wires are correct as shown in Wes' wiring diagram and PCV valve is plumbed properly. Is there something I'm missing in that regard?

It’s not uncommon to find the pcv valve installed backwards.

Main key in your post is that she ran fine until last week. You've indicated you did no mechanical repairs, replacements or adjustments that would have caused the change in behavior. So I think we are now down to waiting for your tests results.

1 - Vacuum gauge readings at idle, acceleration and 2500 RPM.
2 - Fuel pressure reading at carb inlet fitting.
3 - Compression test results.
4 - Leak down check results.
5 - Actual timing behavior static and running.

Anymore guessing at this point is simply guessing.

In a post back on page 1 he said the last thing he did before the problem surfaced was replacing the Voltage Regulator.

Yes, I caught that but he said the regulator change was before the rough running started. If the regulator caused it by severely over or under charging he would have seen the issue on his amp or volt meter and mentioned it.

Main thing now is getting a very good and thorough diagnosis of the jeep's current operating conditions/parameters.

It's been over a year and I thought I'd post an update on this issue as its finally solved. After I tried all the suggestions given here and had no joy, I sent the Carb and fuel pump out to be professionally rebuilt. Not cheap either. Then things got a little busy here on the Southern Border so I didn't have much free time to go to Corpus and install the rebuilt items. Finally got a day to get over there and found my nephew had moved into the house where the jeep was garaged and stored a mountain of his crap in there. I managed to make a path through it and installed the carb and fuel pump. No change from before and just had time to get back to Edinburg.
In December got reassigned to the HQ in Austin so went to Corpus, attached a chain to my tow hitch and the pintle hook and pulled the Jeep along with a pile of my nephews still unmoved crap,out of the garage, then towed it up to Bertram where a guy has a shop that works on all types of MV vehicles. Eventually got it back running perfectly.
Problem was this: The carb rebuilder installed the wrong size and gauge springs in the carb. Rebuilt it with correct parts and no problems.
Didn't mean to ramble for a one line answer.

Good to hear you got her back to running condition. Simple mistakes like those springs can cause a lot of trouble!

lilm38 wrote: Thu Jul 17, 2025 1:22 am I sent the Carb and fuel pump out to be professionally rebuilt. Not cheap either.
Problem was this: The carb rebuilder installed the wrong size and gauge springs in the carb. Rebuilt it with correct parts and no problems.

Glad to hear you got it fixed, and thanks for coming back to close the loop!
Too many posts are left open ended --- (no value to anybody).

It would be helpful to know who did the defective rebuild so we don't make the same mistake.

Be nice to hear exactly which springs were incorrect. I did mention two of the three springs in my earlier replies in this post.

[image]http://willysmjeeps.com/v2/modules/gall ... _1826A.jpg[/image]
[image]http://willysmjeeps.com/v2/modules/gall ... .sized.jpg[/image]
[image]http://willysmjeeps.com/v2/modules/gall ... .sized.jpg[/image]

Here is the shop work order:
Shop Labor MVPA Rate: Performed compression test. All cylinders over 100psi and
within 10% of each other.
Checked ignition timing. The flywheel is installed 180* off or cam is installed 180* out.
Possibly also oil pump drive. Brought #1 to TDC and confirmed balancer is aligned
with timing mark on front cover. Set firing order to correct sequence. Engine now runs.
But backfires out carb indicating a lean condition.
Found fuel pump loose on its mounting. Removed fuel pump, and replaced mounting
gasket. Reinstalled pump. Fabricated new steel fuel line to carb.
Ordered carb kit. Disassembled carb. Found original accelerator pump spring to be to
short.. installed correct springs in carb. Adjused float per service manual.
Reassembled carb.
Adjusted points gap to .010 . set ignition timing to 5* BTDC
set curb idle to 600rpm and idle mixture screw to 1.5 turns out.
Engine now runs smooth. Installed rear drive shaft, and test drove Jeep for about 1
mile. Vehicle accelerates very well. Runs smooth. Labled; spark plug wires, distributor
rotation, and Advance/ retard direction

Common error on carb assembly. Usually the two springs that fit in the two diaphragms will fit on either side of the carb however they are definately not the same length springs and many people mix them up on final assembly. Another common error is placing either spring on the wrong side of the diaphragm. For the M38's YS637S carb the Army's Carb manual TM 9-1826A dtd 1952 in it's assembly diagram Fig. 60 at page 114 shows the spring on the wrong side of one diaphragm which is very misleading. It is the spring item BB that is on the wrong side of the diaphragm.

Thanks Wes. I guess what's irritating is the place I sent the carb off for rebuilding was highly recommended and not inexpensive. I don't want to say the name because It's my belief the error was atypical of their work based on the many positive online reviews received. I'm sure they would have corrected it if I had caught the issue earlier.
On a different note, I wonder why, after all these years of reprinting the original Army technical manuals, any errors that have been identified and confirmed through trial and error, aren't corrected by the publisher? If nothing else, they could include an addendum or place an insert in the appropriate sections. It would avoid mistakes and frustration by anyone not familiar with sites like this who assume, understandably, that the official government manuals are all correct.

Having spent 22 years in The Air Force and Army. Then the last 36 years as a licensed FAA aircraft mechanic and IA I have learned NO government manuals are fault or error free.

The problem with using and relying on Army/Government manuals dated in the early 1950's which were by the 1990's or earlier declared obsolete by the US Army you must always question what you read in them and be wary when following them the problem erupts or cannot be solved. Once the manual is obsolete the Army or Government does not pay any attention to them or their errors.

I understand that the Army wouldn't update the manuals, I meant the publishers who reprint the manuals currently. They could make the corrections fairly easily I would think, by adding a leaflet to the inside cover.