How to Tune Up Your Gas-Powered Lawn Mower

Safety

Before we start the tune-up, we need to think about safety.

Many accidents happen when people don't pay attention to safety. The blade on a lawn mower turns several hundred revolutions per minute. Even a dull blade can cause injury if someone's hands or feet get too close. Never work on a lawn mower without first removing the spark plug wire from the spark plug. With the spark plug wire removed, the engine can't start.

Another safety matter to address is gasoline. Never smoke or have an open flame around a lawn mower, and never refuel a hot engine. These precautions will reduce the risk of explosion.

Tools Needed

Before we begin the tune-up, read this booklet and gather the following hand tools:

• Screwdrivers

• Hammer

• Open-end wrenches

• Feeler gauges

• Needle-nose pliers

• Sockets

• Wheel puller

• Starter clutch removal tool (available for purchase at small-engine repair shops)

• SAE 10 W 30 motor oil

• Oil drain pan

• Funnel

• Siphon hose

• Nut driver

• Spark plug wrench

• Dollar bill

You will also need an engine tune-up kit that can be purchased from your local small-engine repair shop or at most hardware stores.

PERFORMING THE TUNE-UP

Changing the Oil

Let's begin by changing the engine oil. The oil drain plug will be located in one of two areas, depending on the engine model you're working on: under the mower deck or at the bottom of the oil filler tube.

If the plug is located under the mower deck, remove the spark plug wire and place the mower on a raised platform, such as a pair of cement blocks. This will enable you to remove the oil drain plug from under the mower. If the oil drain plug is located under the oil filler tube, raising the mower will not be necessary.

Place the oil drain pan so that it will catch the oil as it drains from the engine. Contact your local fire department or Department of Environmental Quality if you spill any oil. Remove the drain plug and drain all of the old oil from the engine. When all the old oil has drained, replace the drain plug.

The average small engine requires 20 ounces of oil to fill the crankcase. Pour the fresh oil into the filler tube or fill hole. Take care not to overfill the crankcase or spill oil onto the ground. If your mower is equipped with an oil dipstick, wipe it clean and push it fully into the oil filler tube. Wait about 10 seconds and then remove the dipstick and check for the proper oil level. Add oil if necessary. If your engine doesn't have a filler tube, the oil level should be just below the top of the fill hole. Replace any cap or dipstick that you removed.

Removing and Sharpening the Blade

If you haven't already done so, remove the spark plug wire. You'll need to turn the mower on its side to remove the blade, but before you do that, you need to remove the gasoline from the tank. Use a siphon tube or pump to remove the gasoline from the tank and drain it into a red gas can, which is certified for containing gasoline. Once the gasoline is removed, turn the mower over on the side opposite the oil filler tube to avoid oil spillage.

The blade can be held in place by one or several bolts. Use a socket to remove the bolts that hold the blade in place. You may need to lodge a block of wood between the blade and mower deck to keep the blade from turning while it is being removed. Take care when working with the mower blade as the ends may be sharp.

Once the blade is removed, take a close look at the taper at each end of the blade. The bottom of the blade is flat, while the top is at an angle. When you sharpen the blade you'll want to follow this taper to get the proper cutting angle for the blade.

Before you begin sharpening, examine the blade edge to ensure it isn't pitted or damaged. If it is, consider replacing it with a new one, as a badly damaged blade can cause excessive vibration and damage vital engine parts.

If the blade is in good shape, you can sharpen it using either a rotary tool or a bench grinder. The bench grinder is the preferred manner for sharpening the blade. You can lay the blade on the tool rest and accurately follow the taper of the blade. You want to sharpen the blade until the end of the blade comes to a sharp edge like that on kitchen knife. Grind the taper first, then turn the blade over and grind the flat side. This same procedure can be followed with the rotary tool.

Repeat the procedure for the other end of the blade. When the blade is sharpened, replace it in the reverse order it was removed; use a wooden block to hold the blade while you tighten the bolts. Never hold the blade with your hands while tightening the bolts, as injury can occur. Once you have replaced the blade, turn the mower back over on its wheels.

Removing the Blower Housing

The blower housing is mounted over the flywheel (see description below) and contains the starter. It's held in place by three bolts, one on either side at the rear of the housing and one over the spark plug. Remove all three bolts with a half-inch socket and wrench.

Pull the blower housing straight up to remove it from the flywheel starter clutch. Set the blower housing upside down away from your work area and put the three bolts inside so they don't get misplaced.

Removing the Flywheel

The flywheel is the round metal disk with fins on the top that is held in place by the starter clutch. You'll need to remove the starter clutch before you can remove the flywheel. To do this, you'll use the starter clutch removal tool you purchased from your local small-engine repair shop. This tool attaches to the two screw holes on the starter clutch and has a handle that is turned to remove the clutch. You'll need to insert a long-handle screwdriver between the fins on the flywheel to hold it during the removal process.

If you weren't able to find a starter clutch removal tool, you can remove the starter clutch by using a nut-driver-type tool and a hammer. Take care with this method, as the starter clutch contains ball bearings and a thin cover that can be knocked loose. If this happens, you will need to replace the starter clutch.

To remove the starter clutch with this method, place the long-handle screwdriver between the fins on the flywheel to keep it from turning. Place the end of the nut driver on the ear of the starter clutch and tap the end with the hammer, unscrewing the starter clutch in a counter-clockwise direction. Continue striking the nut driver with the hammer until the clutch becomes loose and can be removed by hand.

You'll find a convex washer located between the starter clutch and the flywheel; the flat end rests on the flywheel. It must be reinstalled in the same manner when the starter clutch is replaced.

The flywheel is held to the crankshaft by a shear pin, which is either rectangular or half-moon-shaped. If you look down the crankshaft at the flywheel, you'll see a notch in the flywheel where the shear pin makes contact with the notch in the crankshaft.

To remove the flywheel, attach the wheel puller to the two threaded holes and screw the shaft in until it comes in contact with the top of the crankshaft. Use a wrench to turn the shaft until the flywheel breaks loose. Remove the wheel puller and then slide the flywheel from the crankshaft. Remove the shear pin from the crankshaft and place it on the flywheel and set them aside until it's time to reinstall the flywheel.

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How to Tune-Up Your Gas-Powered Lawn Mower