To Adjust Carburetor From the Unstyled John Deere Model "A" Manual
To secure maximum fuel economy (with tractor pulling load) close load needle "C" to point where engine loses power or backfires; then unscrew needle slowly until required power is developed.
In warm weather this should leave needle "C" open approximately 3/4 to 1 turn on kerosene or distillate and slightly less on gasoline. More opening may be required for starting, for cold weather or for very heavy load. Keep load needle closed as far as possible for best fuel economy.
Adjust idle needle "B" until engine runs evenly. In warm weather this varies from from 1-1/2 to 1-3/4 turns on kerosene or distillate and slightly less on gasoline. This setting may vary on individual tractors, and when made let the idle adjustment alone .
If, when load is released governor closes throttle clear shut, causing uneven running (governor opening and closing) screw the throttle lever stop screw "D" against stop spring until idling is satisfactory. With speed control lever nearly back, engine should idle at 350 to 400 R.P.M.
Note: It is recommended that clean spark plugs be installed before attempting to adjust the carburetor. Fouled plugs may lead to incorrect carburetor adjustment.