How to Tune up Your Old Lawn Mower So It Runs Like New

Updated on September 11, 2019

Example of a mower tune-up

That Lawn Aint Gonna Cut Itself!

Time sure flies! It's summer already and that lawn is out of control. Better trim it... err.. crap, why won't the mower start? Come onnn DON'T DIE ON ME NOW!!!

We've all been there. Winter comes and you toss the old mower in the shed or the garage thinking, "I'll work on it next season" and you retire for many months of hibernation next to the fire. But when next season comes and you poke your head out to the stinging sensation of natural sunlight, the old heap is still there, fossilized from the many months of neglect and no matter what you do, it's not going to start. At most you can expect a sputter or 2, just to tease you like those bloody slot machines do when you first start playing them.

"Hmm... maybe I should go out and get one of those electric whatcha-ma-call'ems".

Hold that thought. As much as the pleasure of long orange chords and the sound of armatures steadily grinding may appeal to you, remember that gasoline mower still loves you. Like a child, it stinks, misbehaves and costs a lot but you can't just give up on something you love when it becomes a nuisance. You have to give it care and nurturing and it will do right by you! With every puff of noxious greenhouse emissions, your mower will give you the joy and wonder that only it can. Your well manicured lawn and occasional sprinkle of shredded newspaper will be a testament to how well you look after your own - and isn't that what we pride ourselves in? Isn't it??? Alright then, let's get to work on that beautiful pile of wreckage!

The first step is to replace everything that is old, used or possibly faulty, just to rule out the chances that your mower's disobedience isn't intentional. Sometimes many months of sitting idly can cause corrosion, oxidation or other forms of degradation to various components making them less capable or incapable of functioning properly.

Checking the Electrical System

Go out and buy a new spark plug. You only need 1 and they are dirt cheap so why risk a fouled electrode or corroded terminal when you can rule that out for a few bucks? Examine the condition of the boot that connects to the spark plug. Make sure the inside is clean. Check the condition of the wire for insulation damage. If the rubber is split, cracking or shows any signs of age/damage, replace it.

A good way to make sure that your electrical system works properly is to unscrew the spark plug but leave it connected to the boot, touch the threads to some metal on your mower and try to start your mower, making sure not to touch the plug tip itself. You should notice the plug arcing if everything is in good condition and working properly.

Cleaning Engine Internals

To make sure you get a good clean burn, you have to first get rid of any deposits, gunk, debris or anything else that might have collected in your engine, either within the cylinder or underneath it (in the crank case).

Changing the engine oil is a given and you should get the highest quality synthetic possible. If the engine has compression issues or is very old and worn, get a level higher in viscosity (ie. go from 5w30 to 10w30 for example). Otherwise, just stick with what's recommended. However before you change the oil there's something else you may want to do while the old oil is still in there.

Go out and buy some Seafoam engine treatment and spray a whole bunch into your old oil. Don't worry, you'll be emptying it soon enough so just go crazy and spray a bunch in. If you have any gas left from the past year, go ahead and spray Seafoam in your gas too. Then see if you can get the mower to fire up long enough to burn through whatever gas is left. This will not only clean the intake tract, valves, cylinder walls and piston but the oil/Seafoam mix will also wash old gunky oil deposits out of the crank case at the same time, so that when you replace it, you get more of it out of your engine. Once you have run the mower for several minutes or run out of gas, you can then empty all the old oil and let that drain. You can even throw some new (cheap) oil in there, swish it around and dump it too, just to make sure all the old sooty stuff is gone. Then put fresh high quality synthetic in.

Cylinder damage can lead to a loss in compression

Checking the Mechanical Systems

Moving parts suffer damage from use and from lack of use. They wear down when used and they tend to cease up when not used. Both issues need to be addressed.

Check the condition of your throttle cable, starting chord, cutting blade, governor if applicable and any other moving parts to be sure they move freely. If they don't, apply solvents and lubrication as necessary until they do.

In addition to what you can see on the outside, the cylinder walls may be scratched from age or poor maintenance practices or both. Scratched cylinder walls will lead to a loss in compression. You can remedy this by buying something called "Engine Restorer" and adding 10% to your engine oil (after it's been replaced). This will restore lost power to your engine. If the compression is bad enough, the engine may not start because of it so this step could make a big difference. Similarly, if everything is ceased up there may be too much resistance for the engine to keep itself running so properly lubricating moving parts will also make a big difference.

Preparing New Fuel

Most people don't think of fuel as part of their tune-up but every bit counts so why not focus some of your attention here as well.

Instead of putting that regular ethanol-infused sauce into your poor engine, fill your Jerry can with some Shell V-Power 91 or any other top tier, high octane gas without ethanol in it. While you're out, buy a tin of pure acetone from the hardware store.

When you get it home, add 10% acetone to the gasoline, mix it thoroughly and pour that into your mower. This will add a small but noticeable HP/Torque boost to your engine by vastly improving volumetric efficiency. It won't knock either because acetone raises the gas' octane number even more.

The green line shows improved power with an acetone blended fuel

Air Induction

Speaking of volumetric efficiency, your air filter may be dirty and full of grass so now's a good time to replace that. If you can find a compatible filter with cotton media or something that breathes better than paper, go for it. This will also help.

Air-Fuel Calibration

If you didn't change your fuel you may still need to adjust your mixture screw. If you did change your fuel to the acetone blended kind, you will DEFINITELY need to adjust the mixture screw by making it more rich. This is because acetone (or ethanol for that matter) have a lower AFR than gas and will bring that number down from 14.7. To compensate, simply turn your mixture screw in the rich direction until the idle quality improves and then gets worse. Then back it off a bit until it sounds good again.

Ready to Rock!

With all of these changes and adjustments, your old mower should not only start but run quite well with good, consistent power. You shouldn't have to do this level of maintenance often if you store your mower properly but even if you do this procedure every year it won't take too long and won't cost that much and your mower will last a very long time.

Happy Mowing :D

Do you think gas engines are worth the trouble to maintain, when compared to electric or push mowers?

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This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.


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