The obvious are choke not fully opening, and restricted air. Let's assume those have been covered.
High fuel level is the most common cause. That can be caused by float out of adjustment, float partially sunk, worn needle/seat, loose seat, trash in the needle/seat.
A simple test for sunk float or needle/seat problems is to leave the fuel on, engine off for a few minutes, see if fuel begins to drip out the air horn.
A test of the float level can be done with out disassembling the carb. You can remove the drain plug, put in a barb fitting or pipe nipple, slip on a piece of vinyl tubing, loop the open end above the carb, turn on the fuel and observe the level. It should be about 1/2" below the top of the bowl.
If all that checks out, proceed to the adjustment process. But, to get a good adjustment, everything else needs to be right first. A sick engine will be impossible to get a good adjustment. The compression needs to be right, valves adjusted, no vacuum leaks, ignition system right (no gas fouled plugs), oil changed (especially if it is contaminated with fuel from running rich), and the thermostat working properly.
With the engine up to temperature, start with the idle speed adjustment. The engine needs to be idled down to 400-500 RPM.
Next set the idle mixture. Not sure which carb you have, probably a Zenith or MS, but the idle mix is backward from modern carbs. Turning the idle screw (the small screw up top, just below the mount flange) in will enrichen the mix, out will lean it. What you are adjusting is the air bleed, so more air, less fuel. Back the screw out until the engine stumbles, slowly back in to the best idle, then about 1/8 turn more in. If turning the screw makes no difference, there is a clogged passage in the carb, it will need to come apart and be cleaned.
Then the main adjustment (if adjustable). The main adjust is the large screw, either on top of the bowl, or underneath. This is a little harder to adjust. With the engine warmed up and running, set the throttle to full governed speed, around 2400 RPM. Manually over ride the governor down to idle by pulling back on the linkage or turning the end of the throttle shaft. Then let go, the throttle should instantly spring wide open, then as the engine comes to speed, slowly close to maintain that speed. If that does not happen, the governor is needing adjustment or repair. If the throttle responds correctly, begin repeating the governor over ride procedure, each time turning the main adjust screw in 1/4 turn. Observe the throttle response. Eventually the engine will balk and hesitate when the throttle opens. At that point, begin backing the main adjust out 1/8 turn at a time, repeating the test until the engine takes throttle without hesitation. Then turn it out an additional 1/8 turn. Watch the exhaust, a single puff of black smoke it what you want to see.
Again, this will only work if the engine can breathe properly (in and out), is healthy, properly tuned and up to temperature.
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Today's Featured Article - Good As New - by Bill Goodwin. In the summer of 1995, my father, Russ Goodwin, and I acquired the 1945 Farmall B that my grandfather used as an overseer on a farm in Waynesboro, Georgia. After my grandfather’s death in 1955, J.P. Rollins, son of the landowner, used the tractor. In the winter 1985, while in his possession the engine block cracked and was unrepairable. He had told my father
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