Farmall Cub carburetion

My 1950 Farmall Cub fouls all four spark plugs with black sooty stuff every four to five hours of running. I would say it is running too rich, but is the needle valve on top of the carburetor an adjustment for rich/lean mixture or something else? I would like to try leaning it out. Thinking a new main metering valve would help or is there something else I am missing here?

Been a while since I have messed with a cub but if I remember right the main jet is not adjustable. The screw towards the top of the carb is the idle adjustment and will not help you much if any. It also works backwards so the more you screw it in the richer it makes it idle.
That all said when was the last time you checked and cleaned the air cleaner oil cup and dumped the water and mud out of it. A clogged up air cleaner can cause that problem

Do you know any history on the tractor? As has it always been this way? Something that has developed over time?

It is rare for a carb to need to be jetted leaner unless it's not the original carb or someone has changed the jetting. Best to look at all the possibilities first.

If it is rich enough to gas foul the plugs, it should also be blowing black smoke, getting bad mileage, possibly even diluting the oil with gas.

Some common causes of running rich are restricted air flow or choke not fully opening. Take the air hose off the carb, look at the choke plate position, go for a test run with the filter disconnected, see if it makes a difference. There is also a mesh filter above the oil cup that needs to be cleaned that is often overlooked.

Other causes can be internal carb problems, like float set too high, float partially sunk, leaking/loose/dirty needle and seat, loose main jet, or clogged air bleed inside carb.

If it is not displaying signs of being too rich, plug fouling can also be caused by having the wrong (too cold) heat range plugs, weak spark, excess idling and low power operation, engine running too cold, or not run long enough to get up to temp. If the tractor isn't a working tractor, sees a lot of idling, a hotter heat range plug will help.

Steve, You about covered it all. Did not know about the screen in the air cleaner. I will run more checks as you suggested. Thanks