How Do I Tune Up My Lawnmower?

Special thanks to Jeff Gorton on the following:

“Keep your mower running like new.”

Mowing is enough of a chore without having to deal with a rough-running, poor-cutting lawn mower. With just a few bucks’ worth of parts and a couple of hours’ work, you can get your lawn mower in prime shape to start the mowing season. We’ll show you how to drain the old gas, replace the air filter, put in a new spark plug, change the oil and sharpen the blade—tasks that will keep your gas-powered mower starting easy, running smooth and cutting clean.

1. Add fresh gas at the start of the season:

Fuel system problems top the list of lawn mower malfunctions. Many of these, like gunked-up carburetors, are often caused by gasoline that’s been left in the mower too long. Although fall is the best time to take preventive measures, you can at least get off to a good start in the spring by replacing the old gas in your tank with fresh gas. Gasoline is highly flammable. Work outdoors or in a well-ventilated area away from sparks and flame. Wipe up spills immediately and store gas in approved sealed containers. To dispose of the old gas, call your local hazardous waste disposal site for instructions. Most mowers have a mesh screen over the outlet at the bottom of the tank. If you can see the screen through the filler hole, use an old turkey baster to suck up dirt and debris that may be covering it.


2.Change your oil regularly:

Changing the oil in your lawn mower takes about 15 minutes and costs less than $5. That’s time and money well spent considering that changing oil at the recommended intervals will greatly extend the life of the engine. Most engine manufacturers recommend an oil change at least every 25 hours of operation or every three months. Older mowers have a fill plug close to the mower deck. Fill this type until oil reaches the threads of the refill hole. Two-cycle engines that use a gas/oil mix for fuel don’t have an oil reservoir on the engine and don’t require oil changes.

Before you drain the old oil, run the mower a few minutes to warm the oil and stir up sediment. Then disconnect the spark plug, drain the old oil and add new. Use SAE – W30 oil (check your owner’s manual). There are two ways to drain old oil: through the filler neck or out the drain plug in the bottom of the engine. It’s quicker and easier to drain the oil through the filler neck if your mower has one. If you have an older mower without a filler neck, locate the drain plug on the bottom of the engine and remove it to drain the oil. Pour used oil through a funnel into a plastic milk jug or other container and label it for recycling.

Whenever you tip a lawn mower up on two wheels to work on the underside, only lift the side with the air cleaner. This prevents oil from running into the carburetor and soaking the air filter. Also, if your lawn mower has a fuel valve, turn it off.

Remember to check the oil level occasionally between oil changes, setting the mower on a level surface. Top it off as needed. Newer mowers have dipsticks with markings that indicate when to add oil. Don’t overfill. Check your manual for instructions to see whether the dipstick should be fully screwed in or just set in when you’re checking the level. If you accidentally add too much oil, follow the procedure to drain some out.


CAUTION: Always disconnect the spark plug wire from the spark plug before reaching under your mower.

3.Add pep with a new spark plug:

Often a new spark plug will make a big improvement in the way your engine starts and runs. Spark plugs are so cheap and easy to install that it’s good insurance just to replace your plug every spring. Check your owner’s manual for the correct spark plug, or take the old plug with you to the store to match it up.

New spark plugs are factory set with a .030-in. gap between the electrodes at the tip of the plug. Inspect the plug when you buy it to make sure there’s a gap about the thickness of a matchbook cover. If there’s no gap, the plug may have been dropped and damaged. Choose another one.

If you don’t own a socket wrench set with a 3/4-in. or 13/16-in. deep socket for changing the spark plug, pick up an inexpensive like the one in the photo above.

4.Don’t suffocate your mower—change the air filter:

Air filters are cheap and easy to replace. Dirty air filters choke the engine, causing it to run poorly and lose power. If your lawn is dry and dusty, check the filter after every few mowings. Otherwise, check it a couple of times during the season. Replace it when it starts to get plugged with dirt and debris. One common test is to shine a flashlight through the filter. If you can’t see the light through the filter, replace it with a new one.

Most newer mowers have pleated paper filters that are either flat or cylindrical, while many older mowers have foam filters. Both types of replacement filters are readily available for $3 to $6 at lawn mower retailers, hardware stores and home centers. Take your old filter along and have your mower manufacturer’s name and model number handy.

In a pinch, you can wash foam filters in a solution of laundry detergent and water and allow them to air dry. But since replacements only cost about $3 – $5, it’s best to just buy a new one. In either case, saturate the foam filter in motor oil and squeeze out the excess before installing it as shown below.

As well, until you get a new cartridge filter (see below) you can clean out a dirty cartridge filter with compressed air. Just be sure blow the compressed air from the inside of the filter to the outside.

5.How to sharpen the blade:

Shut off the fuel feed tap and tilt the lawn mower on its side with the carburetor being on top. This will help prevent your engine from flooding.

Disconnect the spark plug and ground the lead to the engine. Using a piece of 2 x 4 and a clamp (as shown) block the blade so that it can not turn and remove the blade be turning the retaining bolt in a counter clockwise fashion. Note which side of the blade is facing out so that you can put the blade back on the same way.

Use a wire brush to clean off any hard build up of grass from the blade as this can affect the balance of the blade. Inspect the blade closely for any cracks or physical damage. If the blade looks cracked, bent or damaged (see below) – replace it.


With the blade now off of the mower, sharpen the cutting edge to a 30 – degree bevel with a bench grinder. Be sure to wear eye protection and to use light even strokes. Don’t force the blade hard against the grinding stone as that will only cause excessive heat which will lessen the tempered strength of the blade. Dipping the just blade ends in water is a good idea to keep things cool.

To keep the blade in balance be sure to grind off the same amount of material from each end of the blade. If you grind one end of the blade 10 times be sure to grind the other end ten times. Once you have both ends ground sharp and clear of any large indents in the blade edge (greater than 4mm) you can use a bastard file to remove any burred edges from the blade newly sharpened edges.

Place your sharpened blade on a Balancing Cone. If the blade does not sit level on the Balancing Cone – it is out of balance. More material needs to be ground of the blade that is sitting the lowest. Continue to take material off until the blade sits level.

Reinstall the blade and torque the retaining nut to manufactures specifications.