willysmjeeps.com

I'm about to the end of my rope with making this Jeep run/operate correctly.

It starts, runs and idles great. It has really good power and no problem pulling the Jeep up to 55+mph. The problem occurs after 5-10+ minutes of heat soak it is very difficult to start acting like it is flooded. It I let it sit for 3-4 hours it will fire right up on the first bump of the starter. For reference I drove it to work today and parked at 7:30ish, went out at 11:30 for lunch and it fired right up. Drove to lunch and parked, ate 15-20 minutes and came out ...bam...super hard to start. Thought I wasn't going to get it going. Pulling the choke and opening the throttle finally got it going.

I've gone through the list of potential issues and I will try to summarize.

I was having issues with the Jeep running rough after it warmed up -> after replacing the coil it made no difference. Turns out it was the Swiss M151 pointless ignition that I run. I replaced that and it immediately fixed the problem. While trying to solve that problem I thought I had an overheating coil issue (live in SC where it is HOT) so I correctly replumbed the vacuum lines to the distributor so that it is pulling fresh air across the coil. I don't have any issues with it running rough after a long run, only starting issues.

I've rebuilt the carburetor within 6 months and it seems to work fine. I was telling someone that this may be the strongest the Jeep has run in a long time. I do note from time to time that the outside of the carb is wet with gas, float level, seal??? Float was set to the specs.

I've rebuilt the fuel pump in the last year and checked all the filters, lines etc. It is definitely supplying fuel.

This is probably not just an issue that occurs, I hadn't been driving my Jeep much and when I did it would just be warm it up, drive it around the neighborhood and put it away. I've been trying to get it up to 100% to take on a trip.

I thought maybe the issue was the exhaust heating up the fuel line that runs by it, I did a bunch of reading on this site and just decided to try to insulate the line. I put some aluminized fiberglass heat shield material on the line and checked it with infrared gun. It didn't seem to make any difference in the problem. I also run ethanol free which is where you typically see this problem.

The carburetor has the correct isolator between it and the manifold.

Wrong float level?
Should I install a pressure regulator between the fuel pump and carb? Could it be my rebuild somehow was high fuel pressure?

I'm lost and just want to enjoy my Jeep.

1952 M38 running stock 24 distributor (M151 Swiss Pointless), stock carb. All fuel/vacuum/vent lines are properly closed, sealed, etc.

It won't hurt to put a gauge in there and see what the fuel pump pressure is. Also leave it plumbed in there, drive it till its fully at operating temps, turn the engine off and watch the gauge.

M37s are notorious for hard hot starts. Turns out that it doesn't take much pressure to overpower the needle and seat and flood the engine while sitting there hot. I imagine the same can hold true on M38s.

btw if it is truly flooding when sitting hot, starting is should be no choke, full throttle.

Is the heat riser functioning correctly. If it does not close when the engine is warm it will drive up the intake manifold temp and after shutdown with perculate the carb dry and leave you with no fuel in the bowl and vapor locked.

Wes, I’m confused. So is the heat riser valve supposed to be open on normal driving around time or closed? Thanks John jr 1952 M38

jeeptj1 wrote:Wes, I’m confused. So is the heat riser valve supposed to be open on normal driving around time or closed? Thanks John jr 1952 M38

It allows the exhaust to pass near the carb when cold, and away when hot...open and closed, in this case, is hard to define.

The word " Closed" infers heat off. Again I say 90% of answers to questions asked on this site are in my photo albums & in this case in the L134 sub-album:



The top left view is "OPEN" allowing exhaust gasses to heat up the intake manifold. This is the startup position until the engine reaches normal operating temperature.

The top right view is "CLOSED" (heat off) which is the correct position for when the engine has reached normal operating temperature.

The middle view shows the original M38 intake manifold heat riser.

The bottom view shows the CJ heat riser.

Your M38 may have either one so confirm which one you have.

Then operate it by hand and insure it is free to move.

Then observe it's position cold.

Then run the engine until it reaches normal operating temperature and observe it's position hot.

I think the heat riser may be my culprit, I will report back after I investigate.

I have checked in recent times that the valve is free and functioning, I know that it works. However, I don't think that it is opening or closing (whatever you want to call it) when it is heating up. From my recollection the counterweight is always staying in the down position. I never thought much about it since it was free and seemed to run good.

I will run it and heat it up and see if it changes position. If it does not change position I will probably wire it to the open position and verify that it fixes my hard start problem.

If perhaps it is a bad spring is that serviceable with the manifold on the Jeep?

You are mixing up open (Intake Manifolded heated) vs closed (Intake Manifold NOT heated). If you decide to wire it in an acceptable operating position then that position needs to be closed (Intake Manifold NOT heated). Which is placing the lever/weight at 2 O'clock.



You remove the arm/weight after you loosen the pinch bolt then slide the spring out of the slight in the shaft.

Thanks for the great advice! I looked at the riser valve arm and the spring is on the wrong side of the pad that it pushes against, basically keeping the arm locked in the open position with spring pressure. I did note that mine was of the "alternative" variant the way the spring was installed.

I put the spring in the correct position and did a brief test as I was unable to take the Jeep out and drive it up to full operating temperature. I did let it idle and warm up for a few minutes and the arm rose to to the 3 o'clock position (horizontal). Hopefully sometime this week I can get it out and drive it to see if the spring is still strong enough to push the valve to the closed position.

The back story on how mine ended up in this position is unknown. When I rebuilt my Jeep motor (several years ago) my stock exhaust manifold had one of the ears on the exhaust flange broken off. Instead of trying to repair this I simply grabbed another intake/exhaust manifold from one of my M38 parts jeeps that was in good operating condition. I checked to make sure the valve opened and closed freely but don't recall touching the riser or spring.

You don't need to actually start your jeep to test the heat riser spring; I use a propane torch, gently, or sometimes a heat gun, and that spring will move.

Thanks for all the help and guidance! Problem solved. Heat riser is working properly and no more heat soak issues. I tested it on a 90 degree day and had no issues with starting after it was at full operating temperatures. I also verified that the valve was closing properly at the 2 o'clock position. The valve starts in the open position then transitions to the closed position as the spring heats up.