You can accurately adjust the idle on your lawn mower with a tachometer. A tachometer is an instrument that indicates engine speed in revolutions per minute (rpm). A correctly set idle prevents the engine stalling when you reduce the throttle from higher operating speeds and it contributes to greater fuel efficiency and smooth engine rpm transitions. You can adjust lawnmower engine idle speed are with the proper tools, equipment and basic mechanical ability.
Connect the tachometer's electronic pick-up wire to the spark plug wire of the lawnmower engine. Wrap the wire tightly several turns by hand. The engine must not be operating at this time.
Remove the engine air filter screw and the filter from the top of the carburetor, using a flathead screwdriver. Turn the screw counterclockwise and lift the filter off.
Start the engine and allow it to operate for five minutes at half throttle to bring it to its normal operating temperature. Once the engine has warmed, reduce the throttle slowly to idle.
Locate the engine air/fuel mixture screw at the bottom of the carburetor fuel bowl.
Turn the idle mixture screw slowly clockwise until the engine revs begin to fall. Turn the screw counterclockwise until the engine begins to pick up again, then turn it slowly back to a mid-point position.
Locate the engine idle speed screw. The screw is on the top of the carburetor connected to the flat throttle stop-plate. The throttle linkage is attached to the other end of the plate arm making the screw location conspicuous.
Adjust the idle screw clockwise with the appropriate screwdriver to increase idle speed or counterclockwise to reduce it. The correct engine idle speed is 1750 rpm for aluminum-cylinder engines or 1200 rpm for engines with a cast-iron cylinder sleeve.
Replace the air filter and tighten the screw clockwise with a screwdriver.