Look at these spark plugs.

What color is the smoke? Blue smoke comes from burning oil, black smoke from having too rich a mixture from the carburetor. Oil gets burned when the piston rings are worn and not sealing well or the intake valve stems are worn and not sealed. Rich mixture comes from multiple possible sources. An air hose from air cleaner to carburetor with a loose lining when warm will act like a choke. A choke cable not anchored near the carburetor can let the choke close from vacuum and intake air flow. A dirty air cleaner can begin to plug and restrict air into the carburetor. Mud dauber's or wasp nests in the air intake plumbing restrict air flow like a partially closed choke.

The main jet adjustment in the carburetor can be set rich. On my gas MF135 I found it plowed better when running slightly rich at full throttle. In gas power aircraft when wanting full power for take off they run full rich, then when at cruising altitude the pilot has a knob that can lean the mixture considerably for significantly improved fuel economy. Besides exhaust gas temperature (running lean makes for hot exhaust) their is a fuel flow gauge to show the effect of the mixture adjustment.

Gerald J.

Oil gets thinner as it warms up. Thinner oil can sneak past the rings and valve guides. You can try thicker oil as a stopgap in the summer months. 20w-50 perhaps.

Those plugs are the Autolite 295. I bet they have about just 12 hours operating time.
The smoke is not black. It looks whitish or light grey. I really can't see blue in there.
The coolant is steady in level, green and clean, I see no oil on top, even after months of resting.
Same with the oil level it, is not rising. Last year I changed the oil cold after it was sitting all winter. I wanted to see if the first fluid out would be water. It was not, just oil.

I remember my dad would pull the head and grind in the valves. The original tank was rusty and he would be fiddling with the fuel bowl, take apart the carb, blow out junk.
I had the carb rebuilt and an inline fuel filter installed. And the current tank is from a CA, less rusty and lined. He used to change plugs often. The original FB magneto was replaced with American Bosch, I have a new Wico on it now.
So with rebuilt carb and new magneto, it starts with just a stab of the starter rod.
 I have to let off the choke pretty quickly or it will stumble and make black smoke.
The choke rod is flimsy but works.

Hard to tell which parts are original or not. But one issue is the carb air inlet does not exactly match (concentric) to the oil filter outlet. I would have to add about 1/4" spacer
between the carb and intake manifold to get them lined up. So the mismatch is bridged
awkwardly with the intake hose. The oil filter is bolted to the block with 4 bolts, I guess that has to be original and it ain't going anywhere.
Was the Marvel tsx154 offered on the 1948 C? If yes, is there a 1/4" or so thick spacer I am missing? Or maybe a different oil filter housing for Zenith vs MS carbs.

My guess is the intake valves guides are leaking as the primary smoke issue. But the engine is worn, I have bad crankshaft end play for one thing. 70 years old now.

Thanks for you help.

A cold running engine will foul the plugs even when everything else is correct. If it doesn't  have, put a 195 degree thermostat in it and get the engine hot. A hot engine will like a leaner carburetor setting. A leaner mixture will burn cleaner.

I meant air filter housing (not oil filter) not aligning with the carburetor. And there are just two holes on the air filter body, not four like I said.
Anyone know why these don't align?
Were most of the  of the C's equipped with the Marvel-Shebler? My tractor is s/n C59611. The tag says tsx154.

I timed the tractor using strobe light.
Took a while to find the Fire mark (30BTDC) on the flywheel. Never did find the C mark.  I cleaned it and painted the mark. Then checked with the light. I had it advanced too far when I did it by ear. So I was running around with it advanced some. How much, maybe 4 degrees?
No one commented on the carb intake and air filter housing not aligning. I guess Allis
must have drilled the mounting holes in wrong location. So soon I will elongate the mounting holes in the air filter to pick it up some. But it makes me uneasy to do so.

I don't think the timing or air filter has anything to do with the fouled plugs. It does take
a long time to get it warm if all I am doing is driving around. But I did get the tractor hot while cultivating.

I solved my own problem. Found out the air filter has slotted holes to mount to block, and that was the reason it did not align with the carburetor. You can see a lot just by looking.