Engine cuts out at cruising speed

I have a '59 CJ5 with original F-head motor. Once upon a time ago, I posted here that the engine would cut out at cruising speed. Once I am in 3rd gear and cruising at a steady speed and then step on the gas, the engine seemingly dies and then instantly springs back to life. In my original post, I thought it seemed electrical like the ignition was cutting out.

However, I'm now convinced it is fuel. I thought maybe accelerator pump but the symptoms don't fit because it accelerates just fine until I get to cruising speed.

It is an original carburetor but completely overhauled. New fuel pump, but a cheap repop. Could it be fuel pump?

Again, it only seems to happen at high speed. When steady on the gas, everything is fine. But when I push on the gas to speed up, it dies. Let off the gas and it springs back to life and everything is fine after that. It is happening much more frequently now.

 

It appears to shut off, which is why I originally thought it was electrical. But now I think it merely stumbles. Let off the gas and push it again and it comes back and will run at cruising speed.

 

Carb is a replacement that was rebuilt by a professional. I rebuilt my original carb twice but still couldn't get it to run right -- I think the shaft bushings were so worn that it was causing a vacuum leak. This rebuilt carb supposedly had the bushings redone but who knows. I will probably order a kit and rebuild it myself just to make sure everything is OK, such as the float level.

I don't notice any other hesitations, sputtering, etc. other than this. Starts good and idles nice, although it does seem to take a while to warm up.

As for MPH, it seems to happen between 40 and 45 while cruising steady.

I think I'll just tear the carb down and rebuild.

 

It's been a while since I installed it but I believe so.

 

Sounds like the issue I had with mine. Cutting out at high mph around 40 mph I too thought mine was fuel. But no, mine turned out to be the points were bouncing and not closing at high rpm. After I set the point gap and dwell. And was told I had installed the spring incorrectly on my points. Reinstalled correctly and my problem went away.

 

I could replicate my problem revving engine very high in driveway. Sounded like rev limiter kicking in on modern vehicles

 

This thread is very interesting. I have the same EXACT problem with Ted. Same symptoms and same cure when it cuts out. Just let off and it picks back up. I'm running petronix in my jeep. Do you have that as well?

Mine bucks like it is running out of gas even though it has plenty. I too have to let off on the gas and the problem goes away. I'm thinking is a fuel pump issue.

 

To me, could seem more like a clogged filter in the fuel tank, especially if it is progressively worsening. "It is happening much more frequently now."
Try blowing back the fuel line first and see if the problem is reduced, at least for a while.

Easy enough to check at the carb fitting. Two things, pressure and volume(flow). F-head pump should put out 3psi, and fill a cup reasonably fast.

 

A few other pieces of data that I should mention.

1. I am running a petronix just like garage gnome.

2. It does take a while to warm up. When I start it cold, it runs like it is misfiring on one or more cylinders. I let it run for a few minutes and then it smooths out. After it warms up, it runs fine.

3. I have not been able to replicate the problem while sitting still. I have only had the problem at cruising speed, and only after driving it for a while so the engine is at full operating temperature when it occurs.

4. This may be important and perhaps I should have mentioned this first but I just thought of it. I have a small vacuum leak around the wiper motor, which is run off the fuel pump. Next time I drive it, I will try plugging that line to see if this vacuum leak is relevant to the problem.

PeteL, I don't think I have a fuel filter in the tank. I added a fuel filter in the line before it reaches the carb. It is a clear filter and looks clean. The Jeep only has about 300 miles on it since a ground-up restoration including a 100% overhaul of the engine and new fuel tank.

Many thanks for all the help and ideas.

 

"When I start it cold, it runs like it is misfiring on one or more cylinders. I let it run for a few minutes and then it smooths out. After it warms up, it runs fine."

When I've had that happen it was a bad head gasket. Pressurized coolant would seep into one or two cylinders during the night, then burn off after starting.
As a diagnostic clue, leaving the radiator cap loose at night or after each run would alleviate it.

Or, maybe your pertronix has a gremlin and is acting up. I don't trust 'black boxes.' When they go partially bad, the symptoms can be totally illogical.

 

I agree, I'm not a fan of the Petronix for that reason alone. At least with points, problems can be diagnosed and resolved. My original distributor needs rebuilt and something I have not yet gotten around to -- the Petronix is a temporary measure until then.