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That idea of disconnecting three spark plugs sounds odd. If you look around this web site you will likely find a tractor that used that carb. Armed with that information, you could probably find the carb adjustments. Some John Deere 2 cylinder tractors used th Marvel Schebler carbs. To get this engine running, you will likely make the first set of adjustments on a cold engine and then readjust after it warms up. As a general rule, you start with the mixture screws about 1 1/2 to 2 1/2 turns from bottomed out. I don't know if you have a separate idle mixture screw or not. Don't bottom the screws with a lot of muscle or you'll put a groove in the needle valve. Adjust the idle mixture screw for smoothest running - too lean/screw too far in and the engine starts to die - too rich/screw too far out and you will get kind of a "missing" sound. Then adjust the idle stop screw for a low but smooth idle, and readjust the idle mixture screw. For the high-speed mixture, use full throttle up to get the engine's highest idle speed. Adjust the load mixture screw the same way as the idle mix screw, except there won't be an idle stop to mess with.
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