Posted: Wed Feb 08, 2023 10:27 am Post subject: M38 fording system vacuum leak?
I have been searching for a vacuum leak.
Mostly original 1952 M38 with "late" fording system.
Tried carb cleaner and propane in ususal areas. No leak found.
Discovered the vent line to the master cylinder is disconnected.
The other end T's to the wiper at rear of air cleaner.
Could this open line at MC cause a vacuum leak and poor running?
Joined: Oct 02, 2014 Posts: 2009 Location: South Carolina, Dorchester County
Posted: Wed Feb 08, 2023 6:12 pm Post subject:
Hi JeepDaddy,
I thought Wes would've responded by now, so this is just my 2 cents.
I'm pretty sure there's a difference between a vacuum line and a vent line.
I think a vacuum leak on the engine intake side (like the carb base, intake manifold, or PCV) would affect air-fuel mixture or proper operation of the YS-637S carb because it runs on vacuum --- and cause "poor running" problems.
Vent lines, like on the fuel tank, brake master cylinder, and transfer case are connected "up stream" of vacuum (for lack of better description) to the air filter.
Those lines exist yes for waterproofing --- and the proper operation of that particular component, for example, like preventing vapor lock in the gas tank fuel system.
I don't think a vent leaking (or even missing entirely) on the brake master cylinder would cause poor running problems for the engine.
What makes you think you have a vacuum leak?
What's "poor running" symptoms?
Have you connected a vacuum gauge to the intake to see what it tells you?
Thank you RonD2. You are always willing to help and share knowledge. Appreciate it.
This M38 did run and idle with vacuum 18.5 (with slight needle flutter). Had to remove and reinstall manifolds (with new gasket) due to coolant leak at studs. Torqued to 35 ft lbs.
The heat riser rebuilt and working correctly. Fording valves open.
Static timing at 5 degrees, confirmed with timing light. New points gapped to 0.20 and new condenser. Confirmed strong spark and clean air cleaner. Professionally cleaned fuel tank and new fuel filter.
The YS637S rebuilt by ScoutPilot and ran at his shop. No luck for me, so it's a problem under my hood I can't figure out yet. Symptoms:
hard to start, requires long periods of cranking (afraid I'll burn up starter)
will not start with choke (per manual)
only starts after long crank at open throttle
only runs at higher rpms
seems to run better with some choke
if you are not constantly on the gas, rpm's slowly decrease until engine dies
will not idle
Joined: Jun 18, 2009 Posts: 77 Location: North Carolina
Posted: Wed Feb 08, 2023 7:25 pm Post subject:
Maybe you still have a fuel deliver problem to the carb
Have you verified the fuel pump is in good condition and providing enough to carb? Mine had some similar problems to this only to find out small passage in carb was still blocked and not delivering enough fuel when needed, Also one time before that original fuel/ vacuum pump diaphram ruptured and was sending fuel to the crankcase oil Just something to check if not already checked?
Joined: Oct 02, 2014 Posts: 2009 Location: South Carolina, Dorchester County
Posted: Wed Feb 08, 2023 8:10 pm Post subject:
I recommend caution troubleshooting this problem --- by only checking, changing, or confirming one thing at a time --- or the solution can easily get lost or compounded in the shuffle.
My thoughts from long distance not standing there looking at it:
YS-637S rebuilt by ScoutPilot isn't the problem --- so long as the base gasket and diffuser aren't leaking air and the throttle linkages from the pedal and cable are set properly (the throttle butterfly is fully closed at rest).
Also assume your intake manifold was properly mated to the exhaust manifold and both are flat against the block.
18.5 on the vac gauge seems low to me, should be 21 or better.
Diagnostic chart points to valve adjustment or maybe advance the timing a bit (from 5* to maybe 2-3* BTDC).
Spark plugs gap .030?
You say it only starts with "open throttle" --- does that mean wide open throttle?
The manual says to pull the manual throttle cable open 3-4 clicks when starting.
And use full choke with a cold engine. Choke cable and choke butterfly adjusted properly?
Like WillyMan suggested, have you tested the fuel pump for volume and pressure? Specs are in the manual. Many vacuum gauges can do this test. The best hookup is like ScoutPilot uses with the gauge hooked into the carb fuel input line but you need special plumbing to do that. Connecting directly to the pump output will also work.
If the diaphram is blown I think if you pull the crankcase dipstick can you smell gasoline in the crankcase? (If yes, also do an immediate oil and filter change).
I'm not an advocate of starting fluid, but in some cases it can be revealing. On a cold engine and with the crossover tube disconnected, if you put a squirt into the carb throat does it immediately start and run?
Should idle about 600 RPM with throttle adjusting screw set properly. Do you have a tachometer to check what rpm it will stay running at?
Good luck! _________________ 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
As Ron mentioned above the M38 late fording system does include vent and engine vacuum plumbing for it's different modes of operation. Best when troubleshooting vent/pressure/fording/vacuum system issues that you layout all the system drawings on your work bench. They will be very important to your ability to understand each system and isolate each syst and it's components to insure you do not assume or make troubleshooting errors.
The short answer to your question "
Quote:
Could this open line at MC cause a vacuum leak and poor running?
" is NO. It is a full time master cylinder vent line which uses the air filter to filter it's vented air. With the fording side pipe kit installed it will provide dry filtered vent air to the master cylinder and the windshield wipers as long as the driver keeps the water below the vent pipe top opening.
The lines lettered below are the Vacuum lines that are suction side and will cause loss of manifold vacuum.
Normally manifold sealing surfaces leaks would show up with the propane check unless the leak was thru the exhaust system at the heat well for the heat riser. Hopefully everyone inspects their manifold's sealing surfaces for trueness before assembly. _________________ Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100
Joined: Oct 02, 2014 Posts: 2009 Location: South Carolina, Dorchester County
Posted: Thu Feb 09, 2023 10:20 am Post subject:
It's hard to offer long-distance diagnostic advice and I hate to insult folks by asking 64 questions because it's better not to assume anything.
For instance --- I hope he doesn't have the float bowl vent on the YS-637S plugged --- and it's either open or properly plumbed to the crossover tube.
The YS-637S is kind of a finicky carb (vacuum operated, vent plumbing, etc).
I was also thinking about his 18.5 vacuum gauge reading and his elevation above sea level (because he doesn't tell us his location).
If he's at high altitude, what's low for sea level might be normal for him.
I hope he comes back to let us know how it's going. _________________ 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
Thanks for your help. I have studied the fording systems diagrams and confirmed the open vent line at MC is not causing a leak. Have capped it and also capped the wiper circuit at location "D" in diagram.
The vacuum gauge is connected to the inlet manifold (at location "A" in diagram).
This M38 did run and idle since the manifolds were reattached. The carb was sent to ScoutPilot due to a stumble off idle. He was great and provided a video of it idling, running with no stumble on his engine. So the problem must be under my hood or something happened in shipping.
Now needs wide open throttle to start (after extensive cranking) and will only run at higher rpms. When you let off the gas, the rpms slowly decrease until it dies.
Plugs gapped to 0.030.
As RonD2 recommends, I don't want a random approach to this mystery.
Will check fuel pressure (but it did run and idle prior to carb rebuild).
I have assumed a vacuum leak, but sure can't find it. I am running out of ideas.
A vacuum leak will cause a leaning of the mixture but a too rich mixture setting, for example a stuck float. Will not idle well or quit but run reasonably well at higher RPM's. A lean mixture will often stall and stumble.
It looks more like you are fighting a too rich issue.
What were your vacuum gauge readings at idle, on acceleration and at high RPM? Read your M38 TM 9-8012 troubleshooting Vacuum issues page 80. _________________ Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100
Thanks Wes and all that have replied. I do try hard to follow the trouble shooting guide in TM 9-8012.
The spark plugs are Champion XMJ-14 and look pretty good, slight tan color on electrode with minimal carbon.
I can't provide current vacuum readings, because it barely runs and does not idle.
However, prior to the carb rebuild (which was for a stumble off idle) my idle vacuum was 18.5 with slight needle flutter. Vacuum dropped to 4 with rapid acceleration and jumped to 25 when decelerated quickly.
SccoutPilot recommended advancing the timing from 5 degrees to 7-8 degrees due to slower burning modern fuel. But without it running for the timing light, it was a subjective clockwise turn of the distributor.
On another note, is it possible to remove the brass nut on top of the distributor and hook up my analog dwell meter/tach to the negative terminal of the coil using an alligator clip? Or do I need the specific military adapter probe for that connection?
Joined: Oct 02, 2014 Posts: 2009 Location: South Carolina, Dorchester County
Posted: Thu Feb 09, 2023 8:07 pm Post subject:
Hi JeepDaddy,
I re-read your posts from the top down again just to be sure.
If I understand correctly, the last thing you did between the jeep running good at idle (except for the stumble off idle) was to remove and replace the carb?
Nothing else?
If that's correct, and assuming the ScoutPilot rebuild is good, then the problem must either be the carb or how you installed it?
Assuming good base gaskets with no air or fuel leaks, are you certain the throttle and choke linkages are properly set?
If you did other things waiting for the carb to be rebuilt and returned, they might bear looking at again first?
Just a thought.
"Slower burning modern fuel"?
I always thought that higher octane gas burns slower (and longer) than lower octane gas.
But speaking about gas......any chance you got a bad load?
Good luck! _________________ 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
RonD2, thanks for taking time to review posts and help with this mystery.
While the carb was off for the rebuild, I removed fuel tank for professional cleaning by my local radiator shop. While the tank was out, I replaced the fuel sender and new stainless steel fuel filter. Goal to give a freshly rebuilt carb clean and filtered fuel.
New carb base gasket with diffuser. Cooper rivets drilled out prior to install.
Confirmed manifold bolts and carb base tight.
No other changes.
It would help to see photos of your current manifold, carb installation and your left side vacuum plumbing and the heat riser.
Have you removed the carb and inspected the intake for any left behind obstructions? _________________ Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100
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