Joined: Dec 02, 2015 Posts: 63 Location: Cold Lake, Alberta, Canada
Posted: Sat May 03, 2025 5:37 pm Post subject: M38Cdn Carb Dilemma
Thought I'd try this community to help troubleshoot my carb issues. Here's the situation:
1. Jeep wouldn't idle when coming out of hibernation but "ran when parked".
2. After talking to the PO, it was determined that the existing carb rebuild was over 10 years old but still worked OK last Fall.
3. I had a NOS kit from Brian Asbury which I installed after blowing out all the ports etc with carb cleaner and compressed air.
4. I set the float to 1/4" with a homemade cardboard gauge across the carb top with the float resting on the needle and spring.
5. Air idle screw set to 1 turn out.
6. I checked the delivery from the fuel pump and it seemed to fill a mason jar easily while cranking.
Still wouldn't idle but seemed to run fine with the choke almost completely closed but very rich as you can well imagine. The throttle response is very crisp when it does run.
Here's where I really started grasping at ideas including a few more tear downs and full blow outs and even resorted to swapping back to the original needle and seat and re-adjusting the float height.
I even ran it through the ultrasonic cleaner to see if that would help.....nada.
I'm outta ideas here folks. I'm no stranger to carb work and I've tried all my tricks to no avail.
I do have a parts carb that I may clean up and try...can't hurt. _________________ Pete Phillips
1952 M38 Cdn W/Winch 52-31658
1953 M100 Cdn 53-71069
MVPA #36514
Pete, You did not mention what manual you are using for your carb work. The Army did publish TM 9-1826A dtd 12/1952 Ordnance Maintenance Carter Carbs. This is the only manual I know of specific to the YS carbs. If you are using this manual care with the figure 60 is needed because it reverses the correct position of the metering rod diaphram spring. Also the throttle rod ball often gets lost during rebuilds and overlooked. Not idling without cloing choke points to overly lean which the metering rod ball can cause if missing. _________________ Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100
Joined: Dec 02, 2015 Posts: 63 Location: Cold Lake, Alberta, Canada
Posted: Sun May 04, 2025 7:44 am Post subject:
Hi Wes,
I did find your download and will dig into that in detail and redo the carb again.
Just to add to this conundrum, I took another old carb and while retaining all the old components and after giving it the same cleaning treatment, the jeep ran exactly the same. Is it possible for the fuel pump to somehow overpressure what the needle and seat can hold back?
I'll report back....wait out _________________ Pete Phillips
1952 M38 Cdn W/Winch 52-31658
1953 M100 Cdn 53-71069
MVPA #36514
The fuel pump pressure range is in the M38 Operator's Handbook TM 9-8012 or the old version TM 9-804. _________________ Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100
Joined: Dec 02, 2015 Posts: 63 Location: Cold Lake, Alberta, Canada
Posted: Wed May 07, 2025 10:11 am Post subject:
Well I've had some success finally. I'm hoping this will get me moveable for the short term as I just ordered a new kit from Midwest Military.
Going through the TM was absolutely helpful in understanding the different circuit flows and cleaning (again and again) along the flow paths a few more times. I still can't definitively say I did anything different or a visible piece of gunk was dislodged....I'll take the win anyways. One thing I found funny in measuring the 3 metering rods I have (original, new kit and parts carb) and all 3 measure differently at 1.346, 1.319 and 1.356 respectively. Idle is good but it stumbles if I give it too much gas. I have the 1.356 rod in it now and may try the others.
I'm getting mixed opinions on the laminated base gasket...is it supposed to be solid or have 4 slots cut out, or have the V-shaped diffuser below? I'm using the solid one that came with the kit. _________________ Pete Phillips
1952 M38 Cdn W/Winch 52-31658
1953 M100 Cdn 53-71069
MVPA #36514
Joined: Dec 02, 2015 Posts: 63 Location: Cold Lake, Alberta, Canada
Posted: Thu May 08, 2025 7:17 am Post subject:
Wes - Roger that on the metering lengths, we'll see what the new MWM kit has in it for manufactured length. I'm hoping it comes with the diffuser base gasket and 2 thin gaskets...time will tell. And I will definitely go through your album, thanks
4x4M38 - Yes it is a YS637, albeit I believe an older version with the extra web running down the side.
Update:
I took this rig out for a test ride and as it got some miles on it, it is running great with crisp throttle response now. Again I'm perplexed as to what the actual solution was but more leaning towards a win from from many different angles _________________ Pete Phillips
1952 M38 Cdn W/Winch 52-31658
1953 M100 Cdn 53-71069
MVPA #36514
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