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willysmjeeps.com :: View topic - Backfire and Hesitation
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Backfire and Hesitation
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lilm38
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Joined: Mar 25, 2006
Posts: 45
Location: Austin, TX

PostPosted: Sun Mar 24, 2024 12:31 pm    Post subject: Backfire and Hesitation Reply with quote

Hi All,
Vehicle is an M38, 24v, Carter YS Carb, Dual AC Fuel/Vac Pump. I've been trying to get this to run correctly but no joy. It hesitates, stalls, emits a gunshot decibel backfire on acceleration. So far I've:
Rebuilt carb twice with Debella's kit,
Fuel pump was rebuilt last year and works fine,
Dist has no cracks, burnt contacts and wires appear ok,
Coil is US made and checks out ok,
Plugs are a few years old from Military vendor but less than 20 miles on them, same with plug wires,
Fuel tank is new and clean and I'm using only non ethanol fuel.
Ive attached a link to a video. I figure its better than trying to describe the issue. I'm deployed on the US/MEX border so I can only get up to Corpus to work on it sporadically whenever I get Leave so I will implement any suggestions as time allows.

Also, whatever the outcome is, I will definitely post a follow up so anyone with the same issue will see what does and doesn't work and what the cause of the problem was. It is, in my opinion, frustrating and pretty rude to ask people for help and then at the very least, not give them a final update and a thank you
Thanks, Steve.

https://youtu.be/DtTW1nowKyc
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Pierre
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Joined: Jun 01, 2021
Posts: 61
Location: Brome Lake, Qc, Canada

PostPosted: Sun Mar 24, 2024 2:53 pm    Post subject: Reply with quote

Hey Steve,
One probable cause is the plug wires from distributor were not at their correct cylinders
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Pierre H.

full restoration in progress
1952 M38 MC60228
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lilm38
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Joined: Mar 25, 2006
Posts: 45
Location: Austin, TX

PostPosted: Sun Mar 24, 2024 3:18 pm    Post subject: Reply with quote

!!!! It was driving fine 3 weeks ago. Then it started popping and stalling and surges in power while I was, of course, taking a date for a Sunday drive. Barely made it back to the garage. But prior to that, no issues.
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wesk
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Joined: Apr 04, 2005
Posts: 16365
Location: Wisconsin

PostPosted: Sun Mar 24, 2024 3:20 pm    Post subject: Reply with quote

The most common source of that problem is a carb rebuilt using the old assembly illustration which shows the diaphragm and spring (items BB & DD) in the incorrect positions for the metering rod diaphragm which is also often confused with the accelerator pump diaphragm & spring (items X & Y) placement as well. Also missing discharge check ball (item H) oftens ends up with back firing..


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Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100

Mjeeps photo album: http://www.willysmjeeps.com/v2/modules.php?set_albumName=Wes-Knettle&op=modload&name=gallery&file=index&include=view_album.php
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Mike_B
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Joined: Dec 10, 2017
Posts: 338
Location: Southern Maryland

PostPosted: Sun Mar 24, 2024 6:53 pm    Post subject: Reply with quote

Another simple thing to check is the fuel filter if you have one...if it's partially clogged it will make the Jeep buck and kick as it gets fuel and looses fuel.

Mike B Smile
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Mike B
1953 M38A1 Brush Truck
1952 M38
1951 M100 Trailer
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RonD2
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Joined: Oct 02, 2014
Posts: 2004
Location: South Carolina, Dorchester County

PostPosted: Sun Mar 24, 2024 7:25 pm    Post subject: Reply with quote

Offering troubleshooting advice from long distance can be difficult at best.

I'll usually ask what was the last thing you did before the problem appeared --- but it seems you might have done more than a few things about the same time that could camouflage it?

Probably not related, but I notice in your video that your hard fuel line from the pump to the carb runs close to the exhaust manifold.
Could be mistaken, but recall an Army MWO that changed the route to move it away from the manifold to help avoid vapor lock.

Maybe I missed it, but also notice only one vacuum line coming from the pump? Where's the other line?

And is the PCV installed in the correct flow direction?

Yes, fix the carb internally if not certain about it.
Double-check the spark plugs firing order.
All 4 spark plugs look like they burn correctly (not fouled)?
Have you done a compression test?

You didn't mention but assume valves are adjusted, carb was fine-tuned, timing set?

Good luck!
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Ron D.
1951 M38 Unknown Serial Number
1951 M100 Dunbar Kapple 01169903 dod 5-51

“The only good sports car that America ever made was the Jeep."
--- Enzo Ferrari

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wesk
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Joined: Apr 04, 2005
Posts: 16365
Location: Wisconsin

PostPosted: Sun Mar 24, 2024 9:58 pm    Post subject: Reply with quote

I just watched your video. I noted a significant stumble on acceleration but I only hear one backfire when you were agressively pumping the throttle at the end of the video.

As Ron mentioned above you have several abnormal routings in your plumbing.

Also I don't see any results posted for a compression check and a leak down check. If you have never done either check just google the tests and you'll see how. I would not take any action at this point without accurate physical test results for the following tests:

1 - Rnning fuel pressure indication.
2 - Running Vacuum readings at idle, during acceleration and at 2500.
3 - Engine standard compression test results.
4 - Engine leak down check results.

The service bulletin Ron mentioned repositions your fuel pump line to the carb away from the exhaust manifold heat.





This illustration should help with ID'g plumbing issues.

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Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100

Mjeeps photo album: http://www.willysmjeeps.com/v2/modules.php?set_albumName=Wes-Knettle&op=modload&name=gallery&file=index&include=view_album.php
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lilm38
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Joined: Mar 25, 2006
Posts: 45
Location: Austin, TX

PostPosted: Mon Mar 25, 2024 3:42 am    Post subject: Reply with quote

Thanks to all for the responses. I will do the recommended actions when I get back to CC. I am aware of the fuel line issue and have been planning to make the change away from the manifold but the potential for vapor lock shouldn't occur at engine start up?
The PCV valve is, I believe, directing flow in the right direction as shown in the manual but will confirm.
Will check plugs.
Will do compression test.
Will check timing
Will install an in line fuel filter
The last thing I did before this started to happen was replace the Regulator. I havent done some of the modifications because in the 25 years Ive had this Ive only driven it on short hops around wherever it was I was living at the time. I signed up and paid fees for a few of those cross country convoys but for one reason or another wasn't able to go. I'll get there though.
Will update asap.
S
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lilm38
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Joined: Mar 25, 2006
Posts: 45
Location: Austin, TX

PostPosted: Mon Mar 25, 2024 3:50 am    Post subject: Reply with quote

One other thing. I did take note of the correction Wes made on the spring placement when rebuilding the carb. Would the spring size have anything to do with this? There were two sizes supplied of the shorter springs in the kit. The long spring supplied was one.
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RonD2
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Joined: Oct 02, 2014
Posts: 2004
Location: South Carolina, Dorchester County

PostPosted: Mon Mar 25, 2024 10:57 am    Post subject: Reply with quote

lilm38 wrote:
The last thing I did before this started to happen was replace the Regulator.

What "regulator"? Fuel pressure?
The M38 doesn't use a fuel pressure regulator.
You can do a fuel pressure and volume test to see if your pump is operating correctly.

Or did you change the Voltage Regulator (VR)?
If the VR is putting out low voltage to the coil that could certainly affect ignition and I'd expect you might find 4 fouled spark plugs.
Did you measure voltage?

No need for a second fuel filter if you have a good M38 filter inside a clean tank.
Why add another point of failure?

I mentioned the MWO fuel line re-route to avoid vapor-lock only because I noticed it in your video.
It's obviously not an issue on a cold start, but as you're in Texas, the heat could be a future problem.

And yes, I believe the spring size inside the carb is important.

Let us know what you discover.
Good luck!
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Ron D.
1951 M38 Unknown Serial Number
1951 M100 Dunbar Kapple 01169903 dod 5-51

“The only good sports car that America ever made was the Jeep."
--- Enzo Ferrari

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lilm38
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Joined: Mar 25, 2006
Posts: 45
Location: Austin, TX

PostPosted: Mon Mar 25, 2024 1:32 pm    Post subject: Reply with quote

Ron,
Thanks for the additional info. Yes, I replaced the Voltage Regulator.
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wesk
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Joined: Apr 04, 2005
Posts: 16365
Location: Wisconsin

PostPosted: Mon Mar 25, 2024 8:36 pm    Post subject: Reply with quote

When troubleshooting your jeep do not overlook the value of my Photo Albums. The link is displayed at the bottom of each of my posts. I have over 1000 illustrations that are able to answer easily 80-90 % of questions asked here and on facebook.











These photos should pass along why I always recommend folks utilize their free access to my photo albums as well as our web site's free Downloads page for special manuals and such. Like TM 9-1826A dated 1952 for the YS carb and TM 9-1826A dated 1952 for the fuel pumps.
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Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100

Mjeeps photo album: http://www.willysmjeeps.com/v2/modules.php?set_albumName=Wes-Knettle&op=modload&name=gallery&file=index&include=view_album.php
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lilm38
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Joined: Mar 25, 2006
Posts: 45
Location: Austin, TX

PostPosted: Tue Mar 26, 2024 7:34 pm    Post subject: Reply with quote

Thank you Wes. Based on your photos it confirms I did install the correct spring sizes for the designated diaphragms. I set the float distance at 1/4" measured between the bottom center of float and the rim of the float bowl. I will check these items again to be 100% certain, along with the other recommended tests, when I get back up to CC.
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RonD2
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Joined: Oct 02, 2014
Posts: 2004
Location: South Carolina, Dorchester County

PostPosted: Tue Mar 26, 2024 8:30 pm    Post subject: Reply with quote

lilm38 wrote:
.....when I get back up to CC.

Curious ---> what (or where) is "CC"?
Your location says Austin.

Until you get back, right now my bet is on the VR. Smile

Take care out there!
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Ron D.
1951 M38 Unknown Serial Number
1951 M100 Dunbar Kapple 01169903 dod 5-51

“The only good sports car that America ever made was the Jeep."
--- Enzo Ferrari

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4x4M38
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Joined: May 30, 2014
Posts: 3459
Location: Texas Hill Country

PostPosted: Wed Mar 27, 2024 8:01 am    Post subject: Reply with quote

Corpus Christi!
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Brian
1950 M38
MC11481
http://www.willysmjeeps.com/v2/modules.php?set_albumName=album372&op=modload&name=gallery&file=index&include=view_album.php
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