Lawn Mower Starts Then Dies

Lawn Mower Starts Then Dies

There may be lots of reasons a lawn mower won't start, but your lucky, your mower is telling you clearly what the problem is.

 

The Lawn mower starts then dies? The most common reason a lawn mower starts and then dies, is because the carburetor is dirty. Other possible causes include:

  •  
  • Stale/Dirty Gas
  • Faulty Choke
  • Gas Filter Blocked
  • Gas Tank Blocked
  • Gas Line Blocked

Cleaning the carburetor and draining the gas tank will fix the problem, it's all covered. I have divided this guide into two pages in the interest of page speed, strap yourself in.

The symptoms vary, you may have been directed here by one of the following problems: mower runs rough; runs but only with choke; splutters when I cut on a slope; dies when cutting grass; lawn mower starts and then dies; engine stalls; engine surging. All of these conditions are consistent with a gummed/contaminated carburetor and or bad gas. This is such a common problem. Last season I received a ton of questions about carburetor cleaning. So, over the winter I recorded a complete video how to repair course on the subject.

 

 

The course is aimed at the beginner and the process of cleaning your carb is broken down into easy to follow tasks. You follow along side me, I start right at the beginning and work my way through your problem step by step. I cover popular engines like Briggs & Stratton, Honda and Kohler. By the end of this course, you'll have save some $$$, learned a ton of new skills and you'll have a sweet running mower.

 

Briggs & Stratton carburetor surging repair

Honda carburetor surging repair

 

Mower Won't Stay Running

The reason it starts and dies is because the engine gets a shot of gas from the choke operation (or primer bulb) - that's enough to get it running, but the blockage in the carburettor starves it of additional gas and then the engine stalls.

 

Cleaning the carburettor and fresh gas will usually solve the problem, however if the gumming is bad, you'll need to swap out the carburettor.

 

Tools You'll Need

Here's a short list of tools you'll find useful to complete the task of cleaning your mower carburetor. These tools aren't essential, but they do make the whole job a ton easier, you'll need:

Sockets and ratchet - Used to remove the carburetor from the mower. A good flexible set with a wide range of tools will pay for itself very quickly.

 

Gas and oil siphon - Super useful tool for removing gas and engine oil. Handling these chemicals can be messy and spills are common. The siphon makes the job look easy, no more removing oil drain bungs, just siphon the oil out through the dipstick hole. This is one of my favorite tools because it saves time and it's mess free.

 

Carburetor cleaner - Recommended as it does a pretty good job at cleaning the carburetor. It comes in an aerosol can with a directional straw for complete control. It's specially formulated to remove gumming and varnish deposits on carburetor parts.

 

Fuel treatment - Every small engine owner should use gas treatment. Most people don't know gas goes off and gas left in small engines can cause real problems as you already know. Using a gas stabilizer will keep the gas in your mower and your gas can fresh for up to two years.

 

You can check out all these tools on this page "Carburetor repair Tools", the pictures link to Amazon where you can check the prices.

 

mower carburetor gumming

Gumming

This carburettor is too bad to clean. 

mower carburetor

Replace

Replacing is easy and the right repair for a badly gummed carburettor.

What's Gumming?

Modern fuels are mixed with alcohol, which is ethanol. The ethanol attracts small amounts of moisture from the atmosphere, this isn't a problem when the mower is being used regularly.

Unsurprisingly the complaint is most common in the spring, when over the winter months the gas has evaporated and left a gummy type deposit. The sticky gunk blocks up the tiny ports and passages of the finely balance carburetor. 

 

Gas Stabilizer

You can prevent this problem by using a gas stabilizer, I use a product called Sta-bil Storage, check the price here on Amazon. It's a lot cheaper than a new carburetor.  Mixed with the gas it will stop gumming and keep the gas fresh for up to 2 years.

 

 

The process is simple, mix the bottle of stabilizer into a can and fill your gas tank, run the engine a while to get the treated gas throughout the fuel system. You don't need to use it all season, just use it towards the end of the season, put it top of your winterizing checklist, so next spring it's pull and mow.

 

 

Check out my "how to winterize your mower guide", I cover everything you need to know.

The gas stabilizer won't clean your carburetor, to do that job we need carburetor cleaner.

Ride-on mower battery jumping

Sludge

This carburettor is repairable, gumming is a carburetor killer.

gas stabilizer

Stabilizer

Using a stabiliser at the end of the season will save you time, stress and money.

What's A Carburettor?

Carburettors are found on all small gas powered motors. The function of the carburettor is to mix air and gas together. Although lawn mower small engines are simple, the carburettors are quite precise bits of kit.

 

They're designed to mix the air and gas to a ratio of 14.7 parts air to 1 part fuel, commonly known as AFR (Air Fuel Ratio). This ratio offers optimum performance. When this ratio is changed, the mower won't run right.

 

A blockage in the fuel system or faulty carburettor can cause the engine to run lean (lack gas), and a faulty, worn out carburettor or blocked air filter, can cause an engine to run rich (too much gas).

 

Common issues with carburettors are: stale fuel; water in fuel tank; tank blocked; fuel cap faulty; fuel lines blocked; fuel filter blocked; carburettor float needle gummed up. Repairing is possible but can be hit and miss. On the upside carburettors are cheap and easy to fit.

 

You may find this page helpful, "New mower carburettors", it lists quality replacement carbs for the most popular mower engines. The pictures link to Amazon where you can check price and availability.

 

Often you can't tell by looking at a carburettor if it faulty or not. So if you clean your carb and she's still not purring, swap her out. 

Choke

A Choke is a metal flap that literally chokes off air entering the carburettor. This creates a richer fuel condition which is just what a cold engine needs. The richer fuel mix helps counteract the dense oxygen rich cold air.

 

As the engine warms up, moving the choke lever to off, opens the flap again. However if the flap doesn't open, the engine will stall as it's now getting too much gas. You can check the choke operation by removing the air filter and moving the choke lever.

 

Your mower may have manual choke flap, auto choke flap or primer bulb.

 

 


mower throttle lever mower carburetor

Choke On

Choke set to full to start a cold engine. The flap should be closed.

mower throttle lever mower carburetor

Choke Off

As the engine warms a little, move the choke to the fast/run position. The choke should be off at this point.

Check that it's moving to the off position.

Ride-on solenoid

Primer

Your mower may not have a choke, it may instead have a priming bulb.  

It has the same end result as the choke plate - gives the engine extra gas for cold engine starts.

Carburettor Jet

Ports are tiny precision drilled holes in a brass tube known as a jet. The jet lives in the center of the carburettor, and as the engine inhales air, it also sucks gas up through the port holes in the jet. 

 

Carburetor jet

Jet

The jet feeds a measured amount of gas to the engine.

If the port holes block, they restrict gas flow which causes a mower to stall, lack power or just not start.

Fuel Bowl

 

All carburettors will have a fuel bowl. The bowl is a reservoir of gas which stands ready to feed the engine as needed. The carburettor jet sucks the gas from the bowl, through the small port holes. 

 

You'll find your fuel bowl behind the air filter, it's a distinct bowl like shape, and you shouldn't need to remove any other parts to gain access. Check this out, I wrote a complete guide on "Bowl cleaning".

 

Mower fuel bowl

Grit

Dirt in the bowl is common and cleaning it will often have you back mowing.

When replacing the bowl, don't over tighten and be careful not to pinch the bowl seal. 

Float & Needle

A carburettor fuel supply usually consists of a fuel bowl, float and needle. The float is as its name suggests; a float, attached to it is a needle, with a rubber tip.

 

The function of the float is to lift the needle as the fuel level rises in the fuel bowl. When the fuel bowl is full, the needle attached to the float will be pushed against the fuel feed port, sealing it.

 

A worn needle seal can cause either too much gas to flow or too little. I wrote this guide to help you check "Carburettor gas flow".

 

Ride-on valve lash

Needle

Needle and float together control gas flow to the fuel bowl.

Any supply problems here will cause poor engine performance. 

Fuel Bowl Feed Bolt

The bowl will collect dirt and moisture and will need to be cleaned. In some cases you may only need to clean the fuel bowl and fuel feed bolt. Not all mowers have the fuel feed bolt.

 

 

Mower fuel feed bolt

Feed Bolt

Not every fuel bowl will have the fuel feed bolt. It's a hollowed out bolt that has a fuel feed port hole, it feeds gas to the jet. 

If your mower has one, it must be very clean.

Cleaning The Gas Bowl

The bowl collects dirt and moisture that sneaks past the gas filter, and often just cleaning the bowl will solve your problem. However if the grit has entered the jet you'll need to strip down the carburetor and clean it.

 

I use fast acting WD40 Carb cleaner, you can check the price here on Amazon. The WD is good stuff, it gets into all the tiny passage ways and breaks down the varnish deposits.

 

Check out below how to clean the fuel bowl. It's usually held on with one bolt and sometimes that bolt is an important part of the fuel feed system. If your gas is older than three months, its stale, so cleaning the bowl won't make it go. You need to drain the tank, carburetor bowl and fill with fresh gas. If this works out for you great! If not, don't worry, I wrote a simple guide that will help you - "Carburetor cleaning".

 

Ride-on mower starter

Pull Wire

When working on your mower, remove the plug wire.

This prevents accidential starts.

mower gas off

Gas Off

Turn off the fuel tap (if fitted), if not use a grips to gently squeeze fuel line.

If your gas is old, drain the tank. Remove an easy to access gas line or drain out through gas bowl.

Mower fuel bowl Mower fuel bowl

Remove

The fuel bowl lives behind the air filter. It will be fixed to the carburettor by one bolt, usually. 

Honda fit a handy drain bolt, which allows you drain the bowl without removing. Nice!

mower gas bowl bolt mower gas bowl bolt

Clean

Some Briggs and Stratton carburettor bowls are held on by a hollow bolt, its other function is to feed fuel to the main jet.

These hollow fuel feed bolts are prone to clogging.

Not all mowers have this fuel feed bolt. If you have, be sure to clean it. I pluck a wire strand from a wire brush to clean the bolt fuel feed hole.

mower carburetor clean

Spray

If you have some carb cleaner, spray some up the main fuel jet.

mower gas float

Flow

Turn the fuel tap on. Now with the fuel bowl removed check fuel flow.  A good flow should be seen when the float is in the dropped position. 

No flow when the float is in the up position. 

Mower fuel bowl

Clean

Clean and refit the bowl, take care to seat it correctly.

The seal not seen in this picture will usually stay on the the carburettor, sometimes it will come off with the bowl.

mower gas tank

Fresh Gas

If your fuel is fresh turn it on, fit the plug wire and give it her a try.

If your mower still runs poorly - Clean carburettor.

What's A Mower Engine Tune-up?

A tune-up is important to the life of your mower. Doing timely maintenance really does pay off in the long run. Your mower engine should be serviced at least once per season, ideally in the spring. Tune-up kit includes: oil; plug; air filter; fuel filter (if fitted); new blade (optional). 

 

All engines will have a model code and date stamped somewhere. Briggs and Stratton stamp their codes into the metal valve cover at the front of the engine. Kohler have a tag and Honda have sticker on the engine.

 

After you find these numbers, buying the tune-up kit on line is easy. If you can't find the code, no problem, remove the air filter and match it against a tune-up kit listed on line. Most mower engines are common, you won't have a problem getting a tune-up kit to match. 

 

I wrote this simple guide that walks you through the whole process - "How to Tune-up your mower"

 

Related Questions

Related Questions

Why did my lawn mower die? The most common reason a lawn mower dies is bad gas, but a loose plug wire will cause it to stall too.

 

How to clean a lawn mower carburettor? Cleaning a carburettor takes a small amount of patience, some carburettor cleaner and a few tools. Remember to take photos before removing springs, levers and gaskets.

 

  • Remove air filter
  • Remove fuel line 
  • Empty & clean gas tank
  • Remove carburettor
  • Remove carburettor bowl, float & needle
  • Remove and clean the jet
  • Reassemble and mow
Auto Technician and Writer at | Website

John Cunningham is an Automotive Technician and writer on Lawnmowerfixed.com. I've been a mechanic for over twenty years, I use my knowledge and experience to write "How to" articles that help fellow gear-heads with all aspects of mechanical repairs, from lawn mowers to classic cars.