The plugs are 18mm, I'm sure. Set the throttle all the way open for compression test. What do the plugs look like? I'm guessing they are very light color, barely tan. It sounds like it will barely run with the choke on, which means it's very lean.
If the carb is clean, super clean, and the idle mixture screw is back in, and the main mixture screw with the knurled knob is in, can proceed from there. Put the plugs back in, get the engine started, and keep the RPM low. Below 800 is best. Leave the big mixture screw alone, and do the idle mixture first. Need to get the tractor warm, so keep it choked enough to warm up the engine to at least 150F. The adjustment will change if the engine is not warm enough.
Once the engine is warm, and the choke is still on, using a screwdriver, turn the idle mixture screw out. You should be able to find a spot where the engine will idle with no choke. You are richening the mixture at idle with that screw. If the idle speed creeps up, turn it down so it stays below 800RPM and the idle speed screw should be turned in so the throttle plate is near the idle stop tab on the carb.
If the engine won't run without choke, there is a blockage in the idle mixture circuit. Take the carb apart, and insure the tiny passage to the main throat of the carb from the idle mixture screw hole is clear. If it is clear, the engine should idle below 800 without popping by turning the mixture screw out.
Once it runs without choke at idle, set the idle idle speed to 600RPM. Now, turn the idle mixture back in until it starts to stumble, and then turn it back out 1/2 turn. Next is the main mixture adjustment. Turn the tractor off. Turn the knurled knob of the main mixture all the way in. This will close off fuel feed from the main fuel passage. Now turn the knob out 1-1/2 turns. Start the tractor. Let it idle for a bit. Try to advance the throttle. If it stumbles as soon as it goes over 900RPM(likely will), turn the main mixture knob out another full turn to 2-1/2. The engine should now run full throttle. It should accelerate smoothly from idle to full throttle engine speed.
If it doesn't run smooth from idle to full throttle try advancing the idle mixture with a screwdriver a bit out(anti-clockwise), to help the transition from idle to main metering. Once it accelerates smooth from idle to full, leave it on full, and now turn the main mixture back in, until the RPM goes down a bit. Then, turn it back out from that point 1/2 turn. There should be no black smoke at any time. If you get black smoke it's far to rich. Run the tractor for about 3-4 hours doing jobs. Take the plugs out and read them. They should be a medium chocolate color. If they are darker than that, it's too rich, and needs to turn the idle, and main mixture in a bit. If they are light brown, you may need to turn the mixtures out a bit. Run it again and read the plugs. Always make adjustments with a fully warm engine.
