Before adjusting the carb, be sure everything else is properly serviced, like the air cleaner, ignition system, cooling system and thermostat (if equipped), valves adjusted, fuel fresh and clean, filter screen clean and in place, oil fresh and free of fuel.
The rest of the engine needs to be up to par, good even compression, no vacuum leaks, exhaust leaks, or cracked/burned out manifolds.
Last, the carb itself needs to be in good working order. No clogged passages, no flooding, or excessively worn throttle shaft.
With the engine up to operating temperature, choke fully open, set the idle speed first. It is important that the idle is slow enough for the carb to be operating only on the idle circuit, down around 400 RPM.
Next adjust the idle mixture screw, the small top screw. The screw adjusts air, not fuel, so turning the screw in enrichens the mix, backing it out leans it. Turn it out slightly, listen for the engine to stumble, then take it in to best idle, then slightly more rich.
If it makes no difference, either the RPM is too high, the float is too high, or the idle circuit/air bleed is clogged. If so, the carb will need to come apart and be cleaned and inspected.
Next adjust the high speed screw, the larger one down below. Turning the screw in leans the mixture, backing it out enrichens it. This adjustment is more difficult as the engine needs to be under load and running on the main fuel circuit to adjust properly.
To get it in the ballpark, try this. Turn the screw in about a turn, or about 2 turns off seat. With the engine at idle, open the throttle as quickly as possible then return to idle, listen to the response. If the engine stumbles on quick throttle, back the screw out 1/4 turn. Keep repeating the test, adjusting the screw out until the engine will take quick throttle without hesitation. A single puff of black smoke is the goal.
This should get the carb into real close adjustment. You can play with it once it has been out working a while. Different loads and weather conditions have some effect on the mixture needs, so adjust as necessary.
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Today's Featured Article - Tractor Profile: Oliver 70 Row Crop - by Staff. The Oliver 70 was part of the "Fleetline Series" and was manufacturered from 1937 to 1948. The row-crop configuration could have either dual or single front wheels. The Oliver 70 was also offered in the standard, orchard and industrial configurations. The tractors were sold in Canada. An early advertisement states "Power when you want it - power when you need it - power to help put in crops when field conditions are right and time and help are limited. That's the power you'll find in an Oliv
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