When a lawn mower engine runs roughly, irregularly, too fast or too slow, you can adjust the idle speed with just a few turns of some screws. Because Toro uses a 4-cycle Briggs & Stratton engine in most of its outdoor power equipment -- from walk-behind mowers, to riding mowers, to snowblowers and small tractors -- adjustment is fairly universal and works from one model to another.
Position the mower on a flat, even surface. Start the engine and let it run for about five minutes to reach normal operating temperature. Then turn the mower off and remove the air filter with a screwdriver.
Locate the three carburetor adjustment screws on the mower's carburetor. The idler-speed screw will be near the top, the idle-mixture screw in the center and the idler-power screw on the bottom.
Turn the idler-power screw clockwise with a screwdriver until it becomes tight and no longer turns. Once turned all the way clockwise, back it off one full turn or 360 degrees with a screwdriver.
Turn the idle-mixture screw clockwise also with a screwdriver until it does not turn any more or meets heavy resistance. Turn the idle-mixture screw clockwise 1-1/2 turns or 540 degrees.
Start the Toro lawn mower and let it idle for one to two minutes. Rotate the throttle control to the highest speed. Turn the idler-power screw 1/8 to 1/4 turn clockwise with a screwdriver until the engine begins to sputter, counting the number of turns. Then turn the idler-power screw counterclockwise 1/8 to 1/4 turn until it begins to stall, counting the turns. Set the idler-power screw in between those two positions. The engine should run smoothly in this position.
Rotate the Toro's throttle control back to normal. Turn the engine's idler-speed screw clockwise with a screwdriver until it reaches its highest idling speed.
Repeat the same process with the idler-mixture screw, turning it 1/8 to 1/4 turn clockwise until the mower's engine begins to sputter, counting the number of turns. Then turn the idler-mixture screw counterclockwise 1/8 to 1/4 turn until it begins to stall, counting the turns. Set the idler-mixture screw at the midpoint of each position.
Turn the Toro's idler-speed screw counterclockwise with a screwdriver until it idles normally.