Vacuum and timing, weird engine issues

[vforge]

Hello,
Recently bought a 1953 M38A1. Not my first jeep as I also have a ‘63 CJ5. It didn’t really occur to me when I bought the M38A1 but it sounds like a diesel engine. It much noisier than my cj5. I checked the valves and they were adjusted properly. Compression numbers were ok, all between 110 and 125. (Not optimal but not awful).

It also has a weird problem of being difficult to start when it’s been sitting for a few days. The vacuum lines are rather crudely done so I’m wondering if that could be messing with it. I went to test the vacuum at the intake manifold and ran into some weird problem.

with the vacuum line disconnected and the vacuum gauge connected, the engine wanted to immediately start but wouldn’t run with considerable throttle. When it did finally run it sounded like only 2 cylinders were firing and it was smoking. I shut it off, reconnected the vacuum line and it started fine. While it was running, I disconnected the vacuum line and it immediately increased in RPM. I attached the vacuum gauge while it was running and it measured around 15 inHg, in the late timing. I advanced the timing while it was running both with and without the vacuum line attached. It seemed to sound better and the rpm’s increased as well. I don’t have a timing light yet so I can’t properly adjust it. (I will soon.) I still don’t quite understand the whole vacuum system and how it works. But does any of this seem right?

 

[vforge]

M38A1 running

youtube.com

Video of it running

 

[NDT]

Engine in vid sounds great. Mine is a bit clattery sounding also. On the vacuum circuit, the only vacuum use during normal (no wipers) operation should be the crankcase ventilation valve. Engine vacuum is also routed to the fuel/vacuum pump, then the wipers. It’s possible the vacuum pump has a ruptured diaphragm. That could cause issues.

 

[vforge]

Engine in vid sounds great. Mine is a bit clattery sounding also. On the vacuum circuit, the only vacuum use during normal (no wipers) operation should be the crankcase ventilation valve. Engine vacuum is also routed to the fuel/vacuum pump, then the wipers. It’s possible the vacuum pump has a ruptured diaphragm. That could cause issues.

It has a freshly professionally rebuilt fuel/vac pump. The intake manifold has a line that goes straight to a tee which meets with the PCV valve and vacuum pump.

 

[NDT]

I don’t have any ideas other than that you will have to continue diagnostics to find either a vacuum leak or carburetor issues. Mine would routinely flood the carb due to float needle sticking open due to fuel trash. Then it would run rough and smoke too.

 

[Mullaney]

It has a freshly professionally rebuilt fuel/vac pump. The intake manifold has a line that goes straight to a tee which meets with the PCV valve and vacuum pump.

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And, if you want to hunt it down with a little automation - get yourself a can of carb cleaner or a can of starting fluid - and spray it onto the rubber hoses. If the engine speeds up, you found your vacuum leak...

 

[Mrgior31513]

.
And, if you want to hunt it down with a little automation - get yourself a can of carb cleaner or a can of starting fluid - and spray it onto the rubber hoses. If the engine speeds up, you found your vacuum leak...

A small propane torch (not lit, of course) also works well for that. Just put a rubber line over the torch end, crack the bottle until you hear the gas flowing and follow the lines with your 'lead hose'. That method is better for more lines or for finding smaller leaks.

For not starting after a few days, sounds like fuel could be leaking out of your carburetor. If it is, then it's having to refill the carb bowl before it gets enough fuel to start. A slow leak would give it enough fuel to start in a day or two, but then be too empty after that. Could be a number of things that cause that, including needle valve worn/stuck. If it were me, and it hasn't been done in several years, I would just rebuild it or send it out.

 

[vforge]

I’ll try the starting fluid method then the propane method. My smith aircraft torch should work well for that. Thanks.

 

[SilverArrow]

I’ll try the starting fluid method then the propane method. My smith aircraft torch should work well for that. Thanks.

A lot of guys going after similar running issues will do something similar but with the non flammable brakleen (CRC red can). The beauty of this technique is that spraying around hot exhaust or bad distributors won’t result in having your eyebrows burnt off.
With the brakleen method, you are looking for a noticeable stumble in the way it runs.

 

[vforge]

I swapped it over to Pertronix pointless ignition, runs a lot better. The vacuuming system does need to be redone though. I believe the hard starting is a result of bad initial compression.