There may be lots of reasons a lawnmower won’t start, but you are lucky; your mower is telling you clearly what the problem is.
A dirty carburetor is the most common cause of a lawn mower that starts and then dies. 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.
Very often, 5 minutes spent simply draining the gas bowl fixes the problem. It’s all covered here in this post, or if you need video guides, check out “Carburetor gas bowl draining video” and also the “Carburetor cleaning video.”
All popular mower engines are covered, including Briggs& Stratton, Honda, and Kohler. You’ll find them all on the mower repair video library.
Dirty Carb Symptoms
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; die when cutting grass; lawnmower starts and then dies; engine stalls; engine surging.
Cleaning the carburetor and draining the gas tank will fix your problem; it’s all covered here in this post.
Mower Won’t Stay Running
It starts and dies 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 carburetor starves it of additional gas, and then the engine stalls.
Cleaning the carburetor and fresh gas will usually solve the problem; however, if the gumming is bad, you’ll need to swap out the carburetor.
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 the following:
Sockets and ratchet – Used to remove the carburetor from the mower. A good flexible set with a wide range of tools will quickly pay for itself.
Gas and oil siphon – A 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 siphoning 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 be fresh for up to two years.
You can check out all these tools on this page – “Carburetor Repair Tools.”
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 fine-balance carburetor.
Gumming – This carburetor is too bad to clean.
Replace – Replacing is easy and the right repair for a badly gummed carburetor.
Self-Propelled Lawn Mower Powered by a Honda Engine. Three Year Limited Warranty. [More]
That said, if your mower is showing other signs of wear and tear like excessive oil consumption, white smoke, and vibration may be time to put the old girl out to grass, if you know what I mean.
You can prevent this problem by using a gas stabilizer; I use a product called Sta-bil Storage. I’ve included a video below with a link to the stabilizer. Using a gas stabilizer is 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 for a while to get the treated gas throughout the fuel system. You don’t need to use it all season; use it towards the end of the season, and put it on 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 a carburetor cleaner.
Check out the stabilizer video here; it covers it all, mixing and adding.
Sludge – This carburetor is repairable; gumming is a carburetor killer.
Replace – If your carburetor is corroded or very badly gummed, go ahead and replace it. Check out the Amazon carburetor link below, which lists all the most common types.Amazon Lawn Mower Carburetor
Stabilizer – Using a stabilizer at the end of the season will save you time, stress, and money.
What’s A Carburetor?
Carburetors are found on all small gas-powered motors. The function of the carburetor is to mix air and gas together. Although lawn mower small engines are simple, the carburetors 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 carburetor can cause the engine to run lean (lack gas), and a faulty, worn-out carburetor or blocked air filter can cause an engine to run rich (too much gas).
Common issues with carburetors are stale fuel; water in the fuel tank; tank blocked; fuel cap faulty; fuel lines blocked; fuel filter blocked; carburetor float needle gummed up. Repairing is possible but can be hit-and-miss. On the upside, carburetors are cheap and easy to fit.
You may find this page helpful, “New mower carburetors,” which lists quality replacement carbs for the most popular mower engines.
Often you can’t tell by looking at a carburetor if it’s faulty or not. So if you clean your carb and she’s still not purring, swap her out.
A choke is a metal flap that literally chokes off air entering the carburetor. This creates a richer fuel condition, 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 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 a manual choke flap, auto choke flap, or primer bulb.
Choke “On” – The choke is set to full to start a cold engine. The flap should be closed.
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.
Choke Plate – Open (off position)
Your mower may not have a choke; it may instead have a priming bulb. It has the same end result as the choke plate – it gives the engine extra gas for cold engine starts.
Ports are tiny precision-drilled holes in a brass tube known as a jet. The jet lives in the center of the carburetor, and as the engine inhales air, it also sucks gas up through the portholes in the jet.
Jet – The jet feeds a measured amount of gas to the engine.
If the emulsion tube holes block, they restrict air and gas, which causes a mower to stall, lack power, or not start.
All carburetors will have a fuel bowl. The bowl is a reservoir of gas that stands ready to feed the engine as needed. The carburetor jet sucks the gas from the bowl through the small portholes.
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 video on drain and cleaning the gas 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 carburetor 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 or too little gas. Check out this video.
Needle – The 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.
Feed Bolt – Not every fuel bowl will have a fuel feed bolt. It’s a hollowed-out bolt that has a fuel feed porthole; 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 must strip down the carburetor and clean it.
I use a fast-acting WD40 Carb cleaner; you can check the price here on Amazon. The WD is good stuff; it gets into all the tiny passageways 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, it’s stale, so cleaning the bowl won’t make it go. You must drain the tank and carburetor bowl and fill them 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.”
Pull Wire – When working on your mower, remove the plug wire.
This prevents accidental starts.
Gas “Off” – Turn off the fuel tap (if fitted); if not, use grips to gently squeeze the fuel line.
If your gas is old, drain the tank. Remove any easy-to-access gas line or drain out through the gas bowl.
Remove – The fuel bowl lives behind the air filter. It will be fixed to the carburetor by one bolt, usually.
Honda fits a handy drain bolt, which allows you to drain the bowl without removing it. Nice!
Clean – Some Briggs and Stratton carburetor 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.
Spray – If you have some carb cleaner, spray some up the main fuel jet.
Flow – Turn the fuel tap on. Now with the fuel bowl removed, check the 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.
Clean – Clean and refit the bowl, take care to seat it correctly.
The seal not seen in this picture will usually stay on the carburetor; sometimes, it will come off with the bowl.
Fresh Gas – If your fuel is fresh, turn it on, fit the plug wire and give it a try.
If your mower still runs poorly – Clean carburetor.
What’s A Mower 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. The 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 has a tag, and Honda has a sticker on the engine.
After you find these numbers, buying the tune-up kit online is easy. If you can’t find the code, no problem; remove the air filter and match it against a tune-up kit listed online. 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”
Why did my lawnmower die? The most common reason a lawnmower dies is bad gas, but a loose plug wire will cause it to stall, also.
How to clean a lawnmower carburetor? Cleaning a carburetor takes a small amount of patience, some carburetor 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 carburetor
- Remove carburetor bowl, float & needle
- Remove and clean the jet
- Reassemble and mow
- About the Author
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John Cunningham is an Automotive Technician and writer at Lawnmowerfixed.com.
He’s been a mechanic for over twenty-five years and shares his know-how and hands-on experience in our DIY repair guides.
Johns’s fluff-free How-to guides help homeowners fix lawnmowers, tractor mowers, chainsaws, leaf blowers, power washers, generators, snow blowers, and more.