Engine stalls at speed and restarts

[Allen]

I’ve got a 54 M38A1 that stalls while cruising on the highway and within a sec or two powers up again. It has a clean fuel tank, new tank filter new inline filter new fuel pump and rebuilt carburetor.
I can’t reproduce this in shop and runs fine till warmed up and at cruising speed.
Is this a fuel problem or electrical? Any ideas on how to proceed?
Allen in WNC

[LarrBeard]

Is this a fuel problem or electrical? Any ideas on how to proceed?


With all of the new fuel system stuff, let's pretend that it's all good right now.

That leaves electrical. M38A1 - a '54 or so. Does it have Douglas connectors or Packard (MS 33800) rubber connectors? Fuel doesn't cut in and out as abruptly as electrical - fuel generally splutters a bit. Wiggle everything.

M38A1's are boogers to isolate electrical problems because of the convenient service connectors. I'd check all of the connectors around the distributor, the switch on the back of the instrument console and even poke around inside the distributor to see if you have anything loose in there. Since it won't quit when you have it sitting in the shop, road rattle and vibration points a little at a loose electrical thingy somewhere (tech talk).

Let us know what you find - or even don't find.

[gmwillys]

Is your A1 24 volt or converted to 12 volt? If it is converted to 12, there are a plethora of places that if a harness was spliced together, could lose connection.

Another thing to look at first is to check the distributor shaft for any orbital movement. If the bushings are worn, the shaft can literally move away from the points at higher RPM. This will cause the points to not open enough to create a break. The shaft would straighten out after the RPM drops below the RPM until it builds speed back up.

Good luck, and keep us updated!

[bmorgil]

Along the lines of the ignition cutting out you could try this. I would hook up a tachometer to the coil neg. If the tach drops to zero when it cuts out then comes back on again when all is well, it would tell you for sure. If there is no "pulse" at the coil negative, you lost the points, the wire from the points inside the distributor, the positive or the negative to the coil.

[Allen]

Thanks for the suggestions. I am still having the same issue after installing new spark plug wires points and condensor.
I’ve also tightened and sealed vacuum lines.
I checked the distributor shaft for excess play and there is none laterally as expected but about 3/8” up and down. Is that normal or does it indicate a problem?
Once I eliminate the distributor as the problem I’ll get a tach and hook to coil wire to see if I’m loosing fire. I also recently redid some electrical wires with heat shrink connectors. What about the ignition switch? It’s old so could it have a short? I guess anything is possible.
BTW the engine is out of a 56 cj5 which I converted to 12v so there is no old 24v wiring left.

[bmorgil]

Anything is most definitely possible. The fact that it shuts off on you instead of miss firing and stalling, sure does sound like it is losing the spark. The coil is also a prime candidate. It could have an intermittent problem, or it could be overheating and shorting. When it cools it restarts. The coil is oil filled. Are there any signs it may have leaked out? That would definitely cause it to overheat. If you hook up the tach and it drops to zero, there are only a few things left. The ignition switch for sure. The power to the ignition switch and the switch to the coil. And the coil itself. You have replaced everything else.

Recheck that distributor up and down play 3/8" is a bunch. The spec for max looseness in the bearings is .005". That is a bit tight for the end play (up and down) and I suspect few make that spec. On aluminum distributors the spec for up and down is usually .020" This is because the aluminum grows with heat. I do have mine set a .005". If it really is that bad (3/8"), many things can go wrong.

[Allen]

Thanks I’ll start in with the distributor then. Would replacing the bushings fix the 3/8 up and down play or should I just buy a new one from KW?
The coil was installed last year when I converted to 12 v so hope it’s not that. Maybe I’ll just connect the wires behind the switch to see if the switch itself is bad.
Again thanks for the suggestions and will post results of my findings
Allen

[bmorgil]

No the bushings are for side to side play. That must be tight or, the point gap will vary too much as the cams rotate. That is the .005" max spec. This excess radial play, usually shows up as a varying dwell on the meter. If it feels tight side to side its a good one. The end play is set with shims under the collar on the bottom of the distributor. The collar is retained with a pin. It retains the shaft in the distributor. I had to make a custom shim for mine. I really think that you can get away with an 1/8" (.125"). I personally would not run more. It isn't super critical. I have seen a few different motors run with a lot. 3/8" on the other hand could lead to some problems.

[Allen]

Thanks. This gives me renewed hope that this could be the fix I need.