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Lawnmower Won’t Start? Do this.

Lawnmower Won’t Start? Do this.

A lawnmower that won’t start, especially when taken from storage, is almost always due to one problem: bad gas.

Storing a lawnmower in the fall without adding gasoline stabilizer to the fuel tank can cause the fuel to break down and plug the fuel passages. If fixing that problem doesn’t help, there are a few other common maintenance practices to try, as we explain below.

Here’s what to do when your lawnmower won’t start

Replace the gas

Over time (like the six months your lawnmower sat in your garage over the winter), the lighter hydrocarbons in gas can evaporate. This process creates gums and varnish that dirty the carburetor, plug fuel passages and prevent gas from flowing into the combustion chamber.

The carburetor bowl below formed corrosion and deposits during storage, which can easily plug fuel passages and prevent the engine from starting.

Deposits and residue in carburetor bowl

Deposits and residue in carburetor bowl

Ethanol-containing gas can absorb water from the atmosphere, which can lead to phase separation, which occurs when ethanol and gas separate, much like oil and water. Ethanol that has absorbed enough moisture and has sat long enough can foul the fuel system and prevent the engine from starting.

No matter how many times you yank the starter cord and pollute the air with your advanced vocabulary, the lawnmower won’t start if it isn’t getting gas.

In extreme cases, evaporation of lighter hydrocarbons can change the gasoline’s composition enough to prevent it from igniting. The gas may be fueling the engine, but it doesn’t matter if it won’t ignite.

If you neglected to add gasoline stabilizer to the fuel prior to storage, empty the tank and replace with fresh gas. If the tank is nearly empty, simply topping off with fresh gas is often enough to get it started.

On some mowers, you can easily remove and empty the fuel tank. Sometimes that’s more trouble than it’s worth. In these cases, use a fluid extraction pump or even a turkey baster. (We keep them in the Sioux Falls location also)

Clean the carburetor

You’ve replaced the fuel, but your lawnmower still won’t start.

Next, try cleaning the carburetor. Remove the air filter and spray carburetor cleaner into the intake. Let it sit for several minutes to help loosen and dissolve varnish and gums.

On some carburetors, you can easily remove the float bowl. If equipped, first remove the small drain plug and drain the gas from the bowl. Remove the float bowl cover and spray the float and narrow fuel passages with carburetor cleaner.

This kind of “quick-and-dirty” carburetor cleaning is usually all it takes to get the gas flowing again and your lawnmower back to cutting grass.

If not, consider removing the carburetor from the engine, disassembling it and giving it a good cleaning. Be forewarned, however: taking apart a carburetor can lead to nothing but frustration for the uninitiated. Take pictures with your phone to aid in reassembly. Note the positions of any linkages or the settings of any mixture screws, if equipped.

If you’re at all reluctant, visit the servicing dealer instead. Also consider replacing the carburetor altogether. It’s a fairly simple process on most smaller mowers and it’s often less expensive than taking it to the dealer.

Clean/replace the air filter

With the air filter removed, now’s the perfect time to clean it. Tap rigid filters on a workbench or the palm of your hand to dislodge grass clippings, leaves and other debris. Direct compressed air from the inside of the filter out to avoid lodging debris deeper into the media.

Use soap and water to wash foam filters. If it’s been a few years, simply replace the filter; they’re inexpensive and mark the only line of defense against wear-causing debris entering your engine and wearing the cylinder and piston rings.

Check the spark plug

A dirty or bad spark plug may also be to blame. Remove the plug and inspect condition. A spark plug in a properly running four-stroke engine should last for years and never appear oily or burned. If so, replace it.

Use a spark-plug tester to check for spark. If you don’t have one, clip the spark-plug boot onto the plug, hold the plug against the metal cylinder head and slowly pull the starter cord. You should see a strong, blue spark. It helps to test the plug in a darkened garage. Replace the plug if you don’t see a spark or it appears weak.

While you’re at it, check the spark-plug gap and set it to the factory specifications noted in the lawnmower owner’s manual.

If you know the plug is good, but you still don’t have spark, the coil likely has failed and requires replacement.

Did you hit a rock or other obstacle?

We’ve all killed a lawnmower engine after hitting a rock or big tree root.

If your lawnmower won’t start in this scenario, you probably sheared the flywheel key. It’s a tiny piece of metal that aligns the flywheel correctly to set the proper engine timing. Hitting an immovable obstacle can immediately stop the mower blade (and crankshaft) while the flywheel keeps spinning, shearing the key.

In this case, the engine timing is off and the mower won’t start until you pull the flywheel and replace the key. It’s an easy enough job IF you have a set of gear pullers lying around the garage. If not, rent a set from a parts store (or buy one…there’s never a bad reason to buy a new tool) or visit the dealer.

My lawnmower starts, but runs poorly

If you finally get the lawnmower started, but it runs like a three-legged dog, try cleaning the carburetor with AMSOIL Power Foam. It’s a potent cleaning agent designed to remove performance-robbing carbon, varnish and other gunk from carburetors and engines.

Power Foam®

Buy AMSOIL Power Foam

Add gasoline stabilizer to avoid most of these problems

Which sounds better? Completing all these steps each year when your lawnmower won’t start? Or pouring a little gasoline stabilizer into your fuel tank?

Simply using a good gasoline stabilizer can help avoid most of the problems with a lawnmower that won’t start. AMSOIL Gasoline Stabilizer, for example, keeps fuel fresh up to 12 months. It helps prevent the lighter hydrocarbons from evaporating to reduce gum and varnish and keep the fuel flowing. It also contains corrosion inhibitors for additional protection.

Gasoline Stabilizer

Buy AMSOIL Gasoline Stabilizer

I have a five-gallon gas can in my garage from which I fuel two lawnmowers, two chainsaws, two snowblowers, a string trimmer, an ATV and the occasional brush fire. I treat the fuel with Gasoline Stabilizer every time I fill it so I never have to worry about the gas going bad and causing problems.

You can also use AMSOIL Quickshot. It’s designed primarily to clean carburetors and combustion chambers while addressing problems with ethanol. But it also provides short-term gasoline stabilization of up to six months.

Why are There Different Two-Stroke Oil Mix Ratios for Chainsaw Oil

Why are There Different Two-Stroke Oil Mix Ratios for Chainsaw Oil

Swamp mix.

That’s what you call a concoction of two-stroke oil and gas that’s been slopped together with no measuring tools and no regard for engine protection. Your chainsaw or string trimmer could be running on a 50:1 mix. Could be 72:1. Could be 147:1. There’s no telling.

I often made swamp mix back in the day. Like other adherents to the dark art of mixing foul-smelling liquids in a dimly lit shed, I did it for a couple reasons: 1) I didn’t have clearly marked measuring containers at the ready, and 2) even if I had, I wouldn’t have felt like playing chemist with my chainsaw when I’d rather be cutting brush.

I never blew up a saw doing this. But, then again, I’m not a professional logger, so I didn’t cut often.

Some people don’t like mixing two-stroke fuel/oil because it’s a mess. Some hate it because math makes their brain hurt.

What is a two-stroke mix ratio?

The math isn’t that tough to understand. Say your chainsaw manufacturer recommends a 50:1 fuel/oil mix. This simply means you need 50 parts of gas to one part two-stroke oil. Two-stroke engines derive lubrication from the oil mixed directly into the gasoline, unlike your car engine, which lubricates itself with oil housed in an oil sump and circulated by an oil pump.

Complicating things, some equipment manufacturers recommend different mix ratios. While most modern trimmers, chainsaws, leaf blowers and the like call for 50:1, some older equipment recommends 32:1. If you have multiple pieces of equipment with different mix ratios, you have to mix and store multiple cans of fuel. That’s not only a hassle, it’s a situation begging for problems when you accidentally dump 50:1 in your saw that takes 32:1.

This scenario likely isn’t a problem for landscapers, loggers and other professionals who replace their equipment often due to heavy use. But plenty of homeowners still use older equipment that calls for a richer fuel mixture.

Life would be much easier if every manufacturer recommended the same mix ratio, right?

They don’t. But we do.

AMSOIL SABER Professional Synthetic 2-Stroke Oil is tested and proven at any mix ratio up to 100:1. That means you can mix one can of fuel and use it in all your two-stroke equipment. Done. Problem solved. If you want to use 50:1, use that. Or 80:1. For best value, use 100:1. You could mix at 71:1 if you want. It’s up to you.

And, to eliminate the hassles of mixing fuel, we offer bottle sizes matched to certain sizes of gas cans. For example, to mix AMSOIL SABER Professional at 50:1 in a 2.5-gallon gas can, just use an entire 6.4-oz. bottle. Use half the bottle for a 100:1 mix ratio.

That’s the ratio I now use in my chainsaw. As the piston images show, SABER Professional mixed at 100:1 outperforms other two-stroke oils at 50:1, fighting power-robbing deposits and maintaining operability.

AMSOIL SABER Professional mixed at 100:1 delivers better protection against power-robbing deposits than other oils mixed at 50:1.

With SABER Professional you spend less on oil, get better protection and eliminate hassle when mixing fuel. And you never have to make swamp mix again.