On John Deere rear-engine riding mowers, which feature a Kawasaki engine, the carburetor sends the mixture of fuel and oxygen into the engine cylinder to ignite when it comes in contact with spark. Carburetors can be touchy; an aging engine or rough terrain can throw the carburetor out of adjustment, leaving the engine surging or running rough. A thorough knowledge of engine theory is helpful for this task.
Park the lawnmower on a level surface and set the parking brake. Locate the carburetor on the left side of the engine directly under the air filter. On the side of the carburetor are two adjustment screws: the mixture screw that screws into the carburetor body and idle speed screw that screws against the throttle linkage.
Gently turn the mixture screw clockwise until it seats, then turn the mixture screw counterclockwise 1 1/2 turns.
Start the engine and allow it to warm up for two minutes in the "Fast" position. Set the throttle to "Slow" and turn on the tachometer. Wrap the lead coming from the tachometer around the spark plug wire coming out of the front of the engine.
Turn the idle screw clockwise and counterclockwise until the reading on the tachometer is 1550 rpm. Turn the mixture screw clockwise and counterclockwise until the rpms are at the maximum. Turn the mixture screw counterclockwise an additional quarter turn.
Readjust the idle screw to ensure the engine is running at 1500 rpm. Move the throttle from "Slow" to "Fast" several times. The engine should accelerate smoothly; if not, turn the mixture screw clockwise an eighth of a turn.