Joined: Aug 22, 2018 Posts: 9 Location: Rhinebeck, NY
Posted: Fri Oct 06, 2023 4:27 pm Post subject: Hestitates and doesn't develop full power
I have 1955 M38A1. I recently rebuilt the engine down to the block. Rebuilt carb and fuel pump. The engine starts and idles well. I can drive it, however, it hesitates when I press down on the gas pedal.It doesn't reach its full power. I can't get it to go over 30 mph. I cleaned and gapped the plugs, the wires are new. I replaced the points, condenser and coil. Any ideas what I should check?
Thanks....
Joined: Oct 02, 2014 Posts: 2059 Location: South Carolina, Dorchester County
Posted: Fri Oct 06, 2023 6:37 pm Post subject:
Hi wdruiz. Welcome to the forum with your 1st post.
Is your M38A1 and F134 engine bone stock (24 volts, YS-950S carb)?
Do you have a copy of the Army TM9-8014 Operator and Organizational Maintenance Manual for the M38A1?
It's free download in this forum.
On page 79 of that manual, Paragraph 81, is a comprehensive engine troubleshooting guide.
Paragraph f. covers "engine doesn't develop full power" --- but you shouldn't ignore the rest of the paragraph in case there are also other symptoms.
It's most helpful to you, and for other members that want to offer you advice, when you use the manuals and ask pointed questions if you find something that doesn't check out right or seems confusing.
Equally important is that you do your best to directly answer any questions asked by folks trying to help.
The Army manuals for these jeep enable a common frame of reference that helps us all to communicate more clearly.
Hope this helps! _________________ 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
In addition to Ron's statements you should have on your work bench the following test units:
1 - Compression test gage.
2 - Leak-down compression test set.
3 - Vacuum gauge.
4 - VOM
5 - Timing light
6 - Dwell/Tachometer
In addition if your jeep is stock 24V waterproof electrical system
1 - Harness adapter kit.
A pencil and note pad should be there as well to record test results.
As a note on keeping communications on a level field it always help to describe in detail:
1 - Your vehicle. (most folks include their vehicle make/model/year in their signature element for all their posts.
2 - Your problem, Describe in detail and add operating conditions at time of occurrence.
3 - Give your intended viewers access to your 1st name at least to keep the chats friendly. _________________ Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100
Joined: Aug 22, 2018 Posts: 9 Location: Rhinebeck, NY
Posted: Sat Oct 07, 2023 7:01 am Post subject:
Thanks Ron and Wes. I'm new on the forum and appreciate your advice as to the correct forum MO. I’ll update my vehicle’s description under my account/signature as suggested. In the meantime, I have a stock M38A1, everything is close to original, the power train, the stock F-134 engine, the YS-950 carb, etc. It is 24 volt. After my buddy and I rebuilt the engine (finished late spring) we had some trouble with the distributor (the condenser would loosen after a while) after fixing that it ran well. Recently, it started to hesitate in acceleration and doesn’t reach full power on the road. I did read page 79 of the Operators manual. I have a feeling my issue revolves around the distributor.
Thanks,
Wally
Gather up the suggested tools and instruments. Record the results and then post those results here so folks can offer worthwhile suggestions. _________________ Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100
Joined: Oct 02, 2014 Posts: 2059 Location: South Carolina, Dorchester County
Posted: Sat Oct 07, 2023 10:23 am Post subject:
Hi Wally,
Yes, the test results.
And thanks for the clarity that your fresh rebuilt motor did run full power at some point and that this problem just developed.
That's an important bit of information. _________________ 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
Try to think about the three primary causes of hesitation or stumble especially off idle.
1 - Since you did some things to the distributor advance plate by tightening up the mounting of the condenser then the 1st possibility is a stuck advance plate in the distributor. This actually can be troubleshot without opening the distributor up! You just hook up the timing light Check basic timing setting at idle. Then check the 9-11 degrees of advance when you accelerate the engine.
2 - The accelerator pump diaphragm failing or sticking in the carb. This can be checked without disassembly but you'll need a bore scope to see past the 90 degree airhorn bend down into the throttle bore and at idle pop the gas pedal and look for the squirt.
3 - Tremendous vacuum leak at intake manifold. (This is where the vacuum gage comes in handy) (Also a propane torch to sniff the areas all around the intake. You do not light the torch, you simply open it's valve and direct the propane flow at all questionable areas of the intake to block, intake to exhaust and intake to carb connections. Do this while the engine is idling. If there is a leak the propane getting sucked into the leak will suddenly increase the idle speed. _________________ Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100
Joined: Aug 22, 2018 Posts: 9 Location: Rhinebeck, NY
Posted: Sat Oct 28, 2023 5:06 pm Post subject:
Hi fellas,
Ok, I tried a lot of things, but finally found the answer. Yes, Wes the problem was with the distributor but not the advance plate. The cam is worn, on the lobes. The points were not opening properly and it was missing. I polished the cam and reset the points. So far it is running fine. But there is play in the shaft. and I'm wondering how long I can go before I need to rebuild the distributor. It was a strange problem because the engine started fine and idled (though a little rough) but bogged down when accelerating. At least it seems to be resolved for the time being. Thank you again Wes and Ron, I appreciate your help and advice. - Wally
Joined: Oct 02, 2014 Posts: 2059 Location: South Carolina, Dorchester County
Posted: Sat Oct 28, 2023 7:53 pm Post subject:
Hi Wally,
Glad to hear you got it sorted out!
Lots of folks come here for advice and never come back to tell us how it went. Not good.
Just curious --- how do you know the cam lobes are worn?
Did you find a spec and measure it?
Was it pitted or corroded and that's why you polished it? _________________ 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
Yes, Wally, Polishing the distributor cam will not restore worn lobes. Only a new cam will restore it to new dimensions or if one gets adventurous one could weld new material on the camlobes and regrind them to standard! Did you by chance remember to lube the lobes? _________________ Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100
Joined: Aug 22, 2018 Posts: 9 Location: Rhinebeck, NY
Posted: Thu Nov 02, 2023 7:19 pm Post subject:
So, I found that I was not getting spark at one cylinder and intermittently from another. The wires were new as were the spark plugs. That brought me back to the distributor. I noticed that the piece behind the points (the plastic block) was worn even though the point set was new. So, I had someone turn the engine while I watched the cam turn and noticed that the points didn't always open/close. So, I took a chance of polishing the cam and resetting the gap. It seemed to correct the problem. But like i said, there seems to be some wiggle movement in the shaft. Not sure I have a permanent fix.
Joined: Oct 02, 2014 Posts: 2059 Location: South Carolina, Dorchester County
Posted: Fri Nov 03, 2023 9:17 am Post subject:
wdruiz wrote:
I noticed that the piece behind the points (the plastic block) was worn even though the point set was new.
I guess I'm confused.
Polishing the cam on the distributor did nothing to fix the bad plastic block on the points.
Points are relatively cheap, not meant or made to be repaired --- did you replace them?
There's junk aftermarket points on the market.
Finding a quality set can be a challenge.
Standard Motor Products Blue Streak part number AL4556P at your local auto parts store for about $15. _________________ 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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