Farmall Cub 1955, runs fine for about 20- minutes

WE CAN;T HEAR IT RUN.
SEEMS FUEL FILTER CLOGGED
BAD FUEL DOES THAT.
OR CAP VENT CLOGGED
INJECTORS CLOGGED.
FUEL PRESSURE LOW.

It's hard to say if anything you've replaced is contributing to the problem since I don't know the repair history and details.

Assuming everything else checks out fine, I'd definitely check the fuel pressure. Your problem sounds like a fuel delivery issue. Has the fuel filter been changed recently? They become dirty and clogged and restrict fuel flow and reduce pressure.

Fuel pumps go bad and cause problems, so do fuel pressure regulators as well as leaky fuel injectors and other fuel system parts. When fuel pump motors are going bad, they tend to act up when they get warm, which could explain the issues after 20 minutes IF the fuel pump is at fault.

You can get a fuel pressure testing kit from a local parts store; Oreilly Auto rents them for free with a deposit. Check the pressure with the engine off/key on, then with the engine running at idle. And oh yeah, checking the fuel pressure while driving at a higher speed. The fuel pressure could be good but the volume may not be enough at higher speeds, so that's something else to consider for future reference, if needed.

You'll also want to make sure the pressure doesn't drop more than roughly 8 PSI within 5 minutes, after the engine is turned off. These will all be tested against GM's specifications for your car. I'd estimate around 50-60 PSI but that's a estimate and may not be exact.

If you can have those tests run successfully, you'll be closer to narrowing down your issue.

Vapor lock? Fuel filter?

To be quite honest it sounds more like a fire issue than fuel. More times than not when a coil starts going bad the hotter it gets the weaker it gets and sometimes stops firing at all until cooling down, occasionally a plug will do this aswell, but I think your coil is going out.

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1948 TractorData com Farmall Cub tractor information

Check the breather in the fuel cap, run it with the cap loose and see if it is any better.

Slacke off the fuel filler cap when it starts to die, if it picks up again the fuel tank breather is blocked.

sounds like the gas is bad, drain tank, refill will known good gas. You may want to put a fuel cleaner in with the gas, if you let equipment sit around for awhile with gas in it it will tarnish and draw moisture. Hope this helps Tim

sounds like a bad condenser or possibly bad coil, replace the condenser if it has one with points if not sounds like coil.

try the 12volt coil. and remember the positive verses ground . you might have to order one from whitenys.com search that web site , i hope i spelled it right ,dang it sucks being 50 years old and cant remember things. but any ways check that website out okay .