Carburetors regulate the flow of fuel and air into engine cylinders. Fuel and air mixtures are important to your engine. If there is too much air, the engine may not run and you can damage your engine. If there is too little air, your engine will run poorly, smoke, stall and waste fuel. Adjustment to the carburetor on a new Craftsman lawn tractor may not be necessary; they have been preset at the factory. Minor adjustments for fuel, temperature, altitude or load differences may be necessary over time as the mower is used.
Park your lawn tractor on a smooth, flat surface such as a driveway. Turn the engine off and remove the key. Engage the parking brake and disengage the mower blades. Place bricks behind the back wheels of the lawn tractor to prevent it from rolling away. Lift the hood of the lawn tractor and gently push it back until it stops.
Clean the air filter. The air filter consists of two main parts, the pre-cleaner and cartridge. Clean both parts using warm water and mild detergent. Replace the air filter if it cannot be cleaned to prevent dirt from entering your engine.
Check the throttle control cable to be sure it is in the proper position. To check proper positioning, place the throttle lever in the fast position and make sure that the swivel is against the side of the quarter circle. Follow the throttle lever from the dash to the left side of the engine the swivel is a lever located at the end of the throttle. The quarter circle is a metal plate that is angled against the engine in the shape of a quarter-circle. Tighten the cable by loosening the cable clamp screw located halfway down the throttle lever and pull the cable tight. Tighten the cable clamp screw to secure.
Check the choke control. Pull the choke control, located on the top left of the dashboard, to open. Remove the air filter cover, filter and cartridge to look at the carburetor choke. The choke should be closed. If it is not closed, loosen the casing camp screw. Move the choke cable until the choke is completely shut. Secure the casing clamp screw.
Turn the idle mixture screw to the right (clockwise) until it is closed. Do not tighten beyond finger tight (no tools necessary to open and/or close). Turn the idle mixture screw to the left (counterclockwise) 1 ¼ to 1 ½ turns. The idle mixture screw is located on the side of the carburetor just below the air vent.
Start the lawn tractor and allow to idle for 5 minutes so that the engine warms up. Place the throttle control lever on the dashboard in the slow position. Press the throttle lever (located just behind the idle speed screw) against the idle speed screw. Adjust the idle speed screw until the engine reaches 1,200 to 1,400 RPM (if there is no tachometer the engine should sound like it is at full speed and you need to shift to the next higher gear).
Continue to the hold the throttle lever in place. Turn the idle mixture screw to the right (clockwise) until the engine starts to shut off. Turn the mixture screw until the engine "runs rough." An engine is running rough when it hesitates or sounds like it will not keep running. Turn the mixture screw to a point halfway between the two positions.
Hold the throttle lever against the idle speed screw. Adjust the idle speed screw to so that the engine slows to 900 to 1,200 RPM (if there is no tachometer the engine should sound as if you have just shifted and the engine is smoothing out).
Release the throttle lever and move the throttle control lever located on the dashboard from slow to fast. If the engine sounds rough or tries to stall turn the idle mixture screw counterclockwise (to the left) 1/8 turn. Adjust as necessary until the engine accelerates easily.