Gas Lawn Mower Won't Start

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Gas Lawn Mower Won't Start

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Causes

01. Fuel Valve Off

02. Choke Not On

03. Bad Gas

04. Plug Wet/Faulty

05. Plug Wire Off

06. Air Filter Blocked

07. Carburetor Fault

08. Coil Fault

09. Bail Lever/Ignition Fault

10. Flywheel Timing (Shear-key)

11. Sticking Valves

12. Low Compression

Check The Basics

Dirty carburetor is the most common cause of a no start lawn mower - stale or contaminated gas gumms up the carburetor.

 

Doing some basic checks will sometimes solve the problem or point you in the right direction. The basics include: oil level check; fuel level check; fuel tap on; air filter check; plug wire on; choke applied; bail lever applied.

 

 

 

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 10w 30 oil is fine. A mower won't like to be over filled 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".

 

 

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 customer 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 years gas makes a regular appearance in the tank, hey don't worry about it, it happens.

 

Air Filter Check

A blocked air filter will prevent the mower from starting. The air filter needs to be kept clean, check it every 25 hours of use, and replace every 100 hours.

 

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

Its easy for the plug wire to come loose, 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 warn't 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, it covers all the steps and walks you through the process - "How to start a lawn mower".

 

Some mowers do this by way of a choke plate or primer bulb. Some modern mowers offer auto choke, and if you have auto choke, then this section doesn't apply to you. 

 

If your mower doesn't start on the 3 rd attempt it's very likely that the plug is now saturated in fuel - condition known as flooded. The fix is to remove and dry the plug or leave the mower for 30 minutes to dry out, and then try again, this time without choke.

 

Bail Lever Applied

The Bail lever or dead mans lever is a safety feature. It's 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".

 

Check Basics


1 Check

Some mowers are designed not to start if the oil level is low, check and top up if needed.


mower dip stick
mower gas valve

2 On

Not all mowers will have a gas tap, if yours has, make sure it's on.

3 Gas

Fresh gas in the tank?


mower gas tank
mower plug wire

4 Wire

Plug wire on and secure?

5 Air

All engines require the right amount of air, if the air filter is blocked, it can prevent starting.

Try starting without the air filter.


mower air filter

6 Choke

Mowers will have a manual, auto or primer type choke. A cold engine will need the choke ON to start. Too much choke will flood a mower, so if your engine won't start on the 3rd pull, turn choke off and try again.


mower choke
mower primer

Primer

Primer bulb style choke, simply press 3 times before pulling the cord.

7 Bail

The bail lever must be on and held for the engine to run.


Bail Lever

Check Basics


1 Oil Check


mower dip stick

Some mowers are designed not to start if the oil level is low, check and top up if needed.

2 Gas On


mower gas tap on

Not all mowers will have a gas tap, if yours has, make sure it's on.

3 Gas


mower gas tank

Fresh gas in the tank?

4 Plug Wire


mower plug wire

Plug wire on and secure?

5 Air


mower air filter

All engines require the right amount of air, if the air filter is blocked, it can prevent starting. Remove the the air filter and try starting now.

6 Choke


mower choke

Mowers will have a manual , auto or primer type choke. A cold engine will need the choke ON to start. Too much choke will flood a mower, so if your engine won't start on the 3rd pull, turn choke off and try again.

Primer


mower primer

Primer bulb style choke, simply press 3 times before pulling the cord.

7 Bail


mower gas tank

If your mower has an oil filter, then check the oil level again after you test run the engine. This can be done at the end of the tune-up.

Gas

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 buy 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 to 2.5 gallons of gas, and you can use it in all gas powered kit including 2 stroke engines.

 

You can check the price of Sta-bil on Amazon.

 

 

 

Recommended Gas

Small engine manufacturers recommend regular gas or e10, but e15, 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 manufacturers warranty.

 

Digging Deeper

The lawn mower engine is a wonderfully simple motor, they are very reliable and major mechanical failures are rare. To run, it needs three things, fuel/air mix, good spark and compression. Technically it needs a bit more than that, but you get the idea.

 

 

 

Gas Shot

Having checked the basics, and found all OK, it's now time to dig a little deeper. The gas shot will quickly find your problem. In the workshop, I will run a gas shot test. It involves getting fresh fuel direct into the carburetor, and attempting to start it.

 

Get some fresh gas into your carburetor as per guide below, and if your mower starts or tries to, it's likely you have a gummed up carburetor. I wrote a complete carburetor cleaning guide, it walks you through the whole process.

 

If however, after trying the gas shot test, the mower doesn't start or attempt to, it's likely you have a spark issue. And yes, I have a guide for that too, this guide will help you check the spark and trace the fault, check out "Ignition Test".

 

If on the other hand your spark is OK, then it's likely you have a compression or timing issue. I wrote this guide to help you nail down a timing fault, it covers how to check and repair flywheel timing - "Engine kicks back when starting".

 

 

Gas Shot


Fresh

Clean fresh gas needed, remember gas goes stale after one month.


gas can
mower air filter

Remove

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

Shot

Pour some fresh gas into the carburetor, about a cap full. You may have to tilt your mower on its side to get the fuel to flow in. The alternative is buy a can of starter fluid.


Mower engine
Ride-on oil top up

Pull

Attempt to start your mower in the normal way.

Yes/No


There are two possible outcomes -

(1) Mower attempted to start or started - tells us we have a fueling fault, most likely a dirty carburetor, but first we will check the choke. Check out "Choke system".

(2) Mower made no attempt to start - then we've likely eliminated a fueling fault, and the fault will probably be a lack of spark. Check out "Ignition system check".

Ride-on mower oil dipstick

Gas Shot


Fresh Gas


Gas Can

Clean fresh gas needed, remember gas goes stale after one month.

Remove


Gas Can

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

Shot


Mower air filter

Pour some fresh gas into the carburetor, about a cap full. You may have to tilt your mower on its side to get the fuel to flow in. The alternative, buy a can of starter fluid.

Pull


Pull Start Mower

Attempt to start your mower in the normal way.

Yes/No


mower smoking

Possible outcomes -

(1) Mower attempted to start or started - tells us we have a fueling fault.  Check out "Choke system"

(2) Mower made no attempt to start - then we've likely eliminated a fueling fault, and the fault will probably be a lack of spark. Check out "Ignition system check"

 

Choke

Here we will check that 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 and idle, without stalling.

 

 

Choke Types

Some mowers use a choke plate type system and others will have a priming bulb.Your mower won't start if the choke is faulty.

 

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 which opens the choke plate.

 

Priming Bulb - Is a rubber bulb that operates on vacuum, it basically sucks gas up from the tank and squerts it into the carburetor.

 

Test Choke

Remove the air filter, this allows us view the choke plate.

If you have a manual choke control, move throttle lever to full choke position to start a cold engine.

Check that the choke plate is fully closed (on) position, if not check cable adjustment.

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

Primer bulb can be checked by pressing the bulb and looking into the carburetor, you should see the gas being squirted in.

If you found no issues with your choke system, move on to "Gas Bowl Clean".


Mower air filter Mower air filter Mower air filter

Fresh Gas


Mower throttle Mower throttle Mower carburetor

Remove the air filter, this allows us view the choke plate.

If you have a manual choke control, move throttle lever to full choke position to start a cold engine.

Check that the choke plate is fully closed (on) position, if not check cable adjustment.

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

Primer bulb can be checked by pressing the bulb and looking into the carburetor, you should see the gas being squirted in.

If you found no issues with your choke system, move on to "Gas Bowl Clean".