You ask about the carb adjusting screws, the small one on top is the idle mix screw, the lower one is the main.
Before adjusting, make sure the air cleaner is clean (not just the oil cup, but the upper portion also). Be sure there are no vacuum leaks, the fuel is clean and good fuel supply to the carb.
To adjust the idle, get the engine up to operating temp, mag in retard position. Note the position of the screw slot, make small adjustments, remember which way you turned it so you can go back to where you started.
I believe on the Zeniths, turning the idle mix screw IN makes it richer, OUT makes it lean. So, back the screw slightly out until it runs worse, then back in to best idle, then about 1/8 turn more. Just play with it until it idles best, then slightly richer.
On the main adjustment, turning it IN leans the mix, OUT for richer. This is best set under load, which is near impossible to do. The next best is to set it by observing the throttle response.
With the engine running at full governed speed, mag advanced, manually pull the throttle linkage to idle. Release the link and the governor will full throttle back up to speed. While it is coming up to speed, listen to the engine, watch the exhaust. Again, note the position of the mixture screw. Turn it in 1/2 turn, try the throttle response. Keep turning in until the engine stumbles on acceleration. Then start backing the screw out, testing at each adjustment. When it is right, the engine will come up to speed without hesitation. A puff of black smoke is a good indication it is right.
Don't be afraid to play with the adjustments, each carb and engine is different, you will know when it's right. The carb needs to be in good working order to properly adjust. If it's flooding, or has clogged passages, or if the idle screw makes no difference, it will need to be repaired first. If this tractor ever gets worked, a plug check will tell if it's right. They should be clean, not black. If black, it's too rich. Any change to the exhaust will change he mixture. If you put the muffler on you'll need to readjust the main.
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Today's Featured Article - When Push Comes to Shove - by Dave Patterson. When I was a “kid” (still am to a deree) about two I guess, my parents couldn’t find me one day. They were horrified (we lived by the railroad), my mother thought the worst: "He’s been run over by a train, he’s gone forever!" Where did they find me? Perched up on the seat of the tractor. I’d probably plowed about 3000 acres (in my head anyway) by the time they found me. This is where my love for tractors started and has only gotten worse in my tender 50 yrs on this “green planet”. I’m par
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