Backfire and Hesitation

[lilm38]

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

[Pierre]

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

[lilm38]

!!!! 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.

[wesk]

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..

[Mike_B]

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

[RonD2]

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!

[wesk]

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.

[lilm38]

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

[lilm38]

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.

[RonD2]

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!

[lilm38]

Ron,
Thanks for the additional info. Yes, I replaced the Voltage Regulator.

[wesk]

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.

[lilm38]

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.

[RonD2]

.....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.

Take care out there!

[4x4M38]

Corpus Christi!