Lawn Mower Won't Start? | This Is Why!

By: Author John Cunningham. Published: 2018/12/09 at 5:29 pm

This is so FRUSTRATING!!! Pulling and pulling and nothing, what a pain in the Jacksie. This is a very common complaint at the start of the cutting season, and in this post, we’ll find the problem and fix it.

So, what’s the problem with a gas lawn mower that won’t start? The two most likely reasons a gas mower won’t start are a dirty carburetor and bad gas. Other possible reasons include:

Fuel valve off
Choke not on
Plug wet/faulty
Plug wire off
Air filter blocked
Carburetor faulty
Coil fault
Bail lever/ignition fault
Flywheel timing (Broken Shear-key)
Sticking valves
Low engine compression

I know what you’re thinking. Yes, it’s a long list, but doing some basic checks will sometimes solve the problem or at least point us in the right direction. The basics include oil level check, fuel level check, fuel tap on, air filter check, plug wire on, choke applied, and bail lever applied. Let’s take a look at the basics and rule out all the easy stuff first.

Check out this video; it covers all the common easy to fix causes of mower no start.

We cover the complete diagnosis process here in this post; if you need video help, check out “Mower won’t start video.” It walks you through the whole repair step by step. How to diagnose and fix a no-starting mower.

Check Basics

Oil Level – Some mowers won’t start if the oil level is low; this isn’t a flaw; it’s designed that way to protect the engine.

It’s good practice to check the oil level every time you fill the gas tank.

Check on level ground and with all wheels set to the same height. If you need to add oil, 5w30 or 10w30 oil is fine. A mower won’t like to be overfilled with oil either; this can damage the engine, cause it to smoke, and leak oil. Most mowers won’t hold more than .65 of a quart (.6lt) from empty.

Don’t damage your engine needlessly; I wrote a complete, easy-to-follow guide here, “Lawn mower oil level check” or check out the video here.

Fuel Level – Is there gas in the mower? Sometimes, the obvious is the solution, and as Sherlock Holmes might say, “we should check a fact is indeed a fact.”

The operator may have filled the gas tank with what they thought was good gas.

At my shop, I have found many strange concoctions – diesel, water, white spirits, vinegar, and, of course, last year’s gas makes a regular appearance.

Fuel Tap “On” – Not all mowers have a gas tap, but if your mower does, it’s best to check it’s in the “On” position. Check out this guide for more details on gas tap location, or check video here.

Air Filter Check – A blocked air filter will prevent the mower from starting. The air filter needs to be kept clean and dry, checked every 25 hours of use, and replaced every 100 hours.

Tilting a mower incorrectly commonly causes gas to flood the filter. Check out this post for more details, or check the video here.

Most manufacturers fit tool-less air filter covers. Finding the cover is easy; it will be opposite the muffler. Cleaning the filter with compressed air would be nice, but banging on the ground will do just fine. I wrote this DIY tune-up guide, including pictures; it covers everything you’ll need to know – “Lawn mower tune-up.”

Plug Wire On – It’s easy for the plug wire to come loose; it happens all the time. The plug wire usually lives right at the front of the engine, so it’s banging into shrubs and hedges and the like.

It’s the black wire with the rubber boot on the end. Just make sure it’s making good, clean contact with the plug; bad or no contact will give you a no-start.

Choke On – I meet lots of customers who don’t know how to use the choke correctly, and I don’t blame them; likely, they were not shown by the retailer. The choke is used to start a cold engine.

A cold engine needs more fuel than a hot engine, so the choke creates a richer air-fuel ratio. I wrote this easy-to-follow guide on choke use; it covers all the steps and walks you through the process – “How to start a lawn mower.”

All mowers need cold start help, but they don’t all use a manual choke. Some mowers do this by way of a choke plate or primer bulb. Some modern mowers offer auto choke, and if you have an auto choke, then this section doesn’t apply to you.

Starting a Mower

If, after trying this starting procedure, your mower doesn’t start on the 3rd. Attempt, it’s very likely the plug is now saturated with fuel – a condition known as flooded. The fix is to remove and dry the plug or just leave the mower to sit for 30 minutes to dry out and then try again, this time without choke.

Here’s a video showing how to unflood a mower engine.

Here’s a common starting procedure:

1 Oil Check – Some mowers are designed not to start if the oil level is low; check and top up if needed.

2 Gas “On” – Not all mowers will have a gas tap; if yours has, make sure it’s on and there’s gas in the tank. A low gas level may cause hard starting.

3 Choke “On” – Apply choke or use the primer bulb to enrich the fuel ratio for a smooth cold start.

Primer bulb style choke; simply press three times before pulling the cord.

Mowers may have a manual, auto, or primer-type choke. Too much choke will flood a mower, so if your engine won’t start on the 3rd pull, turn the choke off and try again.

4 Apply Bail Lever – The Bail lever or dead man’s lever is a safety feature. Its function is to stop the spinning blade within 3 seconds of release.

It does this by shutting the engine off and applying a flywheel brake. If the bail lever isn’t held, the mower won’t start. Check out “How to start a Mower”

5 Pull Cord – Yank the cord sharply; the engine must turn over sufficiently quickly in order to generate a spark strong enough to ignite the gas. (about 350 rpm)

Bad Gas Causes Most Problems

As you know, bad gas or a dirty carburetor is the most likely reason your mower won’t start. Most fuels today are an ethanol blend, and this is causing clogging of carburetors if left in the mower over the winter.

The alcohol in the ethanol attracts moisture, and the moisture corrodes the carburetor. The best solution is to use a fuel stabilizer; it will keep your gas fresh and prevent clogging for up to two years.

I use Sta-bil gas treatment at the end of the season; it eliminates carburetor gumming and also cleans the fuel system. You simply mix 1 ounce of stabilizer with 2.5 gallons of gas, and you can use it in all gas-powered kit including 2-stroke engines.

You can check out the stabilizer I use here on the “Small engine repair tools” page and check out the video on using a gas stabilizer here.

Recommended Gas – Small engine manufacturers recommend regular gas or e10, but e15 and e85 are not good for your small engine.

The higher level of alcohol causes the engine to run hotter; these engines are not designed to use ethanol fuels. Using them may void your manufacturer’s warranty.

Lets Dig A Little Deeper

The 4-stroke lawnmower engine is a wonderfully simple motor; they are very reliable, and major mechanical failures are rare.

It needs three things to run: Fuel/air mix, good spark, and compression.

Technically, it needs a bit more than that, but you get the idea. One of the three ingredients is missing, and so now we must run some tests to find out which one.

Need more info on the fuel system, carburetor components, and how they work, you can check them out here.

Gas Shot

Having checked the basics and found all OK, it’s now time to dig a little deeper. The gas shot will find your problem. In the workshop, I will run a gas shot test to quickly troubleshoot which system is causing the problem.

It’s simple; it involves getting fresh fuel (gas must be fresh) directly into the carburetor and attempting to start the engine. This test bypasses the carburetor and, in doing so, allows us to condemn the fueling system or eliminate it as the cause of the problem.

The test goes like this:

1 – Remove the air filter cover and air filter; some will be fixed on with screws or wing nuts, and others will just pull off.

2 – Get some fresh gas into your carburetor as per the guide pic; I tilt the mower over a little.

If you can’t get gas to flow into the carburetor, remove the spark plug and use a funnel to get about a cap full of gas into the cylinder before replacing the spark plug ad wire.

3 Pull – Attempt to start your mower in the normal way.

Result

There are two outcomes –

1 Mower attempted to start or started – tells us we have a fueling fault, most likely a dirty carburetor, but first, we will need to eliminate a choke system fault. Check out “Testing the choke system” below.

2 Mower made no attempt to start – then we’ve likely eliminated a fueling fault; the fault will likely be a lack of spark. Check out “Mower won’t start, no spark” post or look at “Checking mower spark” video here.

Testing The Choke System

All engines need extra gas to start when cold; it helps bring the air-to-fuel ratio closer to the sweet spot (14.7:1). If this ratio is off, the engine may not start or find it difficult to start. Here, we will check the choke system is working correctly.

The correct starting procedure for a cold engine will require giving it extra gas to enrich the fuel/air mixture; this helps the engine start smoothly and idle without stalling.

Choke Types

Most choke systems are manually operated. However, the auto choke system is becoming more common. Some mowers don’t use a choke system at all. They instead use a primer bulb to enrich the fuel ration. Pressing the bulb squirts gas into the carburetor.

Manual Choke – Will have a lever and cable to control the choke.

Auto Choke – Won’t have a choke lever or cable; instead, a thermostat mounted close to the muffler heats and pushes on a lever that opens the choke plate.

Priming Bulb – This is a rubber bulb that operates in a vacuum; it basically sucks gas up from the tank and squirts it into the carburetor.

To Test Choke

Remove the air filter; this allows us to view the choke plate. If you have a manual choke control, move the throttle lever to the full choke position as per the picture above. This is the correct position for starting a cold engine. Now check that the choke plate is fully closed (On position); if not, check cable adjustment. Check out the choke troubleshooting video here.

Your mower may have an auto choke. If so, when cold, the choke plate should be closed, as per the picture. If not, move the plate by hand to see if it’s stuck. If sticking, try WD40; if not, replace the thermostat.

The primer bulb can be checked by pressing the bulb and looking into the carburetor; you should see the gas squirting in. If you found no issues with your choke system, check out this guide, “Carburetor cleaning” or check out the carb cleaning video here.

I’ve covered carburetor cleaning previously, and you can check it out here “Carburetor cleaning” or check out the video here. (It’s a mower carburetor, but they’re all similar).