I am (still) working on a 54 Allis Chalmers G tractor for my 83-year-old neighbor. So far, it has been a nightmare, but it is coming around. 

BACKGROUND:

The owner had the engine overhauled two years ago. When he got the G home, it ran for about 30 minutes and died. It would start back up, but die after a few minutes. His son, who knows nothing about four-stroke engines, worked on it and made things worse. After his son gave up, the tractor would not run at all.

MY REPAIRS:

I found that his son had wired the engine backwards. He had even wired the coil backwards. After I got the tractor running again, it would run for 8–10 minutes on a cold start, before stalling out. After the engine is warm, it will only run for 1–2 minutes. 

PARTS REPLACED:

Fuel tank, fuel line, starter motor, battery, carburetor, sediment bowl, coil (internal resister), condenser, points, wiring harness, plug wires, plugs, and cut off switch. 

PARTS REPLACED TWICE:

Rebuilt carburetor, coil (internal resister), points, condenser, carburetor main jet, and plugs. 

DIAGNOSTICS:

I put a spark tester inline and watched the spark on first start up. When the engine died, the spark was okay. I removed the fuel line from the carburetor and the fuel supply was good. I checked the timing of the engine, and it was dead on. I checked the points gap, and it was good at .020

THINGS TO KNOW:

This tractor has seen many mechanics. Someone replaced the cut-off switch with a household light switch. I rebuild the wiring harness with all new wire, due to a ground problem at the starter. It has been converted to 12v, and is why I used a coil with the internal resister. The problem seems to be heat related. The 12v alternator is not charging. I did replace the condenser with a capacitor, just to see, but it didn't help. 

Guys, I am stumped! I am a retired dealership technician with 35 years of line work under my belt. Never have I given up on a repair, but this thing has been a nightmare. I am open to any and all suggestions, and will report back when I find a repair that works. 

Thanks for allowing me to request help on your forums, and I hope you all have a great week. 

and as gas leaves the fuel tank and goes to the carb, you need makeup air to enter the tank. Make sure the tank is vented with the cap. and NOTHING is floating around in the tank that can block the outlet.

Yup.

I have had both of those problems more than once

This is what I have done, so far. 

Checked and cleaned the tank.

Replaced the fuel lines.

Installed an inline spark tester to see how the spark was, when it shut off. (good spark)

Removed the air filter line, no change. 

Tried spraying carb cleaner into the carb when it starts to die, no change. 

Replaced condenser, no change.

I did notice the voltage regulator is reading low volts, with a full charge. Maybe a bad switch? Someone installed a household light switch as the on/off switch. 

I am baffled!! Never in my life. 

Thanks for all the suggestions. 

If I ever figure it out, I will post it. 

David

David has a good idea... Things that change when you get HOT are valve clearance and electronics.... air flow and fuel flow normally are constant..

Please describe exactly where you had the spark tester installed.