M38A1 carburator issue

lake · Post by **lake** » Sun Jun 15, 2014 1:33 pm

Thought I had a coil issue with my 1953 m38a1,so I replaced it with a new one from Kaiser willy,s auto ,but still no change in the rough running of the jeep.It will run good for aboat 10minutes then gets real sluggish running and seems to be missing real bad and then will quite and will not start for about 15mins or so.Could it be a carurator problem like maybe to much fuel or maybe a need of a kit for the carburator.The spark plugs are fairly new so I elemented that,so I am in need of suggestions.thanks Lake

Bearbait · Post by **Bearbait** » Sun Jun 15, 2014 4:50 pm

A couple things to check before throwing parts and money at it.
Check point gap and condenser.
Check your spark plugs after it stalls, plug wet probably a spark issue, plug dry probably a fuel issue.
Check for a plugged vent in fuel tank (plugged gas cap vent will cause fuel issues).
Fuel line vapour lock can be caused by fuel lines running to close to engine hot spots like exhaust manifold coolant lines etc.
If these things fail check fuel pump output and pressure.
I hope this helps you.

Brad

lake · Post by **lake** » Sun Jun 15, 2014 6:00 pm

ok thanks ,I,ll recheck the points ,the plugs are dry and the fuel pump is fairly new,I am thinking maybe the diaphram in the carburator ,but not totally sure.The fuel filter is totally full of gas which seems like to much fuel may be getting to the carburator .but not quite sure if this is normal

Post by **wesk** » Sun Jun 15, 2014 6:58 pm

The most common cause of your symptoms is the third item above. PLUGGED TANK VENT SYSTEM. The very first and simplist test you need to make is remove the gas cap and start and drive the jeep. No more problem unplug the tank vent system. It may be as simple as correcting the position of the cap's fording valve or you may have a non- selectable cap vent. in which case you will need to unplug the vent over next to the pickup and sender.

This is the selectable vent cap which you may have. The other style cap is un-vented.

This is the later M38A1 tank vent. Earlier M38A1 that came with deep water fording capability had a tube connected here that ran to the air cleaner. In either case make sure the vent valve (late system) or the vent line (Early system) is not plugged.

lake · Post by **lake** » Mon Jun 16, 2014 4:10 am

Ok,I checked the gas cap and it is already in the open position,but i will check out the tank fent tonight thanks.Should the gas filter be totally full of fuel,because my is and I don,t remember it always full to the top.(maybe to much fuel possible to the carb ???)

Post by **wesk** » Mon Jun 16, 2014 7:03 am

Fuel pressure and flow are generally a constant and do not change during operation much.

An engine that runs smooth and starts easily then when it warms up runs poorly and then quits and won't restart until it cools down has trouble with something easily affected by heat buildup. Change the condenser and bypass the radio noise filter capacitor found in the main power receptacle for the distributor. Both can fail as they get warm and work fine cool.

http://willysmjeeps.com/v2/modules/gall ... .sized.jpg
This is the radio noise filter.

This is a fabricated eliminator for it.

Also cracks in the distributor block will often expand with heat and create issues.

skyjeep50 · Post by **skyjeep50** » Mon Jun 16, 2014 7:30 am

"Gas filter full to the top" - A stock M38A1's fuel filter is in the gas tank, has a very large surface area and is usually sufficient to flter the fuel - no need for a secondary filter. If your jeep has an aftermarket fuel filter, yes, that might be a thing to check to see if it may be blocked. If the original fuel filter is still in the tank, perhaps it is plugged if there is a lot of crud and rust in the tank and its never been serviced. You have to unscrew the fuel pickup to check. If it is still there and in good condition, ditch the other filter - its not needed.

lake · Post by **lake** » Mon Jun 16, 2014 9:46 am

ok thanks ,I will start with the condenser first,the jeep starts great, sounds great and even exelerates great,when cold, or for the first 10-15mins and then really starts to miss once warm.???

lake · Post by **lake** » Mon Jun 16, 2014 3:20 pm

just remembered the points and condenser was new last year and was picked up at napa auto.But I am not sure if they sold me a 24 volt condenser or a regular 12volt.Would this be a possible issue ,to make the engine break down after warming up.Maybe I should try a 24volt condenser ??

lake · Post by **lake** » Wed Jun 18, 2014 8:32 am

I have replaced the condenser and coil with a new ,but still no change.I removed the spark plugs and they seem to be very black with carbon .The plugs are only 1 year old, so I cleaned them again and the jeep started and ran fine for another 15 mins or so,Well I guess I am still puzzled and My M38A1 still remains in my garage at rest. ????:

Post by **wesk** » Wed Jun 18, 2014 8:53 am

Have you bypassed the radio noise filter capacitor yet?

Birdy · Post by **Birdy** » Tue Jul 21, 2020 10:31 pm

This past weekend during a convoy run, my M38A1 began to run very rough and guickly stalled out just after cresting a long hill, barely allowing me time to get off the road. I could not restart and had to get towed home. Luckily I was less than 10 miles from home.
Thinking this was a failed coil, when I got home I went through my distributor and replaced coil, points and condenser. I also cleaned the plugs, and the jeep fired right up and idled perfectly. I took off again in attempt to re-join the group on their return trip. Well I got about 3 miles down the road and it started missing and sputtering. I turned around and tried to get back home, and didn't quite make it. Had to get towed again!
Over the last couple of evenings I tried my spare carburetor, spare fuel pump, checked for water in gas, went through the distributor at least 4 times, checked air cleaner, checked fuel and vent lines, spark on all cylinders, timing, etc, etc... No luck.
Then last night I came across the post above by Wes related to the radio filter. I changed that out today, and.... the jeep runs perfectly as it did before! I was blown away, but so happy to solve the issue. Thanks Wes!!
I have replaced those filters with new ones in the past, but that is the first time I have ever had one fail. It is a very confusing set of symptoms.
I will be making the direct wire assembly as shown above to eliminate that filter.
Anyway, I just wanted to pass this on so others may save themselves a lot of pain and suffering if they experience the same issue.
Thanks again Wes.

Post by **wesk** » Tue Jul 21, 2020 10:44 pm

Glad it worked out for you!