Sounds like fuel problems, but the carb can only work as well as the rest or the engine will let it.
That it will idle well is a good sign. That says the compression is good.
Before going back into the fuel system, take a look at the ignition system. One of the most common causes of poor performance is a worn distributor shaft bushing which causes the points setting to vary. The shaft must have very little side play. Also check the centrifugal advance. The rotor should turn a few degrees then spring back when released. If it is frozen or slow to return or the springs are missing, the distributor needs to come out, be disassembled and repaired, or replaced with a rebuilt unit.
Look everything over closely. End result, each plug wire should deliver a blue hot 1/4" spark to ground at the plug end of the wire. If there is any doubt about the plugs, replace them. It's too easy to chase a problem that new plugs would have fixed, they can do strange things.
Once you are satisfied with the ignition system, and any other regular maintenance items have been addressed, like adjusting the valves, inspecting the valve train, the charging system and battery are good, (bad battery or nonworking charging system will cause poor spark), then the fuel system can be diagnosed.
Start with the air cleaner. A clogged air cleaner can actually cause a lean condition. Inside the canister, above the oil cup, is a wire mesh that periodically needs disassembly and cleaning. Some are easy, some difficult, but if it's packed with dirt it will have to come apart. If it's been a while or never been cleaned, you can assume it is clogged.
Before going back into the carb, check the fuel delivery. There is a drain plug on the bottom of the carb. With the fuel valve open, get a clean glass, remove the plug and catch the flow of fuel. If this is a gravity flow system, do this with the engine off. If there is a fuel pump check it with the engine idling.
The open plug should give a full flow of fuel. As the bowl empties the flow will slow, but not stop or slow to a drip. If it does, there is a fuel delivery problem. Start back tracking toward the tank. There is sometimes a screen in the elbow at the carb, a screen in the sediment bowl, and possibly a screen inside the tank. If this is a gravity system, and an inline filter has been added, that could be the problem. They don't work well with gravity systems. If there is a fuel pump check it's performance.
Look at what was caught. If it is dirty, water on the bottom, the same will be in the carb. The tank will need to be removed and cleaned. If it is flaking rust, get ready to continue fighting it or replace it.
As for the carb itself, what was it doing before? If it was running better before, revisit what was done. Be sure everything was put back properly. There are many "how to" videos on those carbs, and factory repair sheets and specs online.
If the tractor has been stored with stale gas in the carb, the passages may need to be scraped out with a torch tip cleaner or piece of wire. Dried ethanol residue will not flush out with spray cleaner, it has to be scraped and prodded to get it all out. Also be sure the float level is properly set and the float is not bent or touching the inside of the bowl.
The adjustment for the idle and main jets start at about 2 turns off the seat. Adjust the idle mix with the engine fully warm and idling at a slow RPM, 450-500 RPM. Turning the screw in richens the mix, out leans it. You are adjusting air bleed, not fuel flow. The adjustment is minimal. If it makes no difference the idle is too high or the idle circuit is clogged.
To adjust the main mixture, with the engine at slow idle, suddenly open the throttle and listen to the response. If the engine falters, back the main screw out slightly. if it responds quickly, start turning the screw in, repeating the test until it stumbles, then start backing the screw out, repeating the test until it will take throttle without hesitation. A single puff of black smoke is the goal.
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Today's Featured Article - An Old-Time Tractor Demonstration - by Kim Pratt. Sam was born in rural Kansas in 1926. His dad was a hard-working farmer and the children worked hard everyday to help ends meet. In the rural area he grew up in, the highlight of the week was Saturday when many people took a break from their work to go to town. It was on one such Saturday in the early 1940's when Sam was 16 years old that he ended up in Dennison, Kansas to watch a demonstration of a new tractor being put on by a local dealer. It was an Allis-Chalmers tractor dealership,
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