Mower Starts But Stalls, This is Why
It can be frustrating when you are ready to cut your lawn and your lawn mower starts then stalls immediately. This article explains why it happens and how you can fix the problem.
Gasoline powered lawn mower engines need a proper mixture of air, fuel and spark inside the cylinder to create combustion. A lack of spark, a restricted airflow and too little (or too much) fuel may cause the lawn mower to run poorly, or not run at all.
Good News! You can eliminate spark as a cause of your lawn mower starting and dying immediately because if there was no spark it would not start at all. That leaves air and fuel issues as the likely causes.
Check the Air Filter
To rule out airflow problems, quickly check the air filter for signs of dirt, oil or other debris. Replacing a dirty air filter will allow more air into the carburetor and may solve the problem quickly.
A clogged air filter can prevent air from flowing into the engine, causing it to stall. Check the air filter and clean or replace it if necessary.
Check the Fuel Filter
Some small engines are equipped with a fuel filter located on the fuel line between the gas tank and carburetor. If your lawnmower is equipped with a fuel filter, take a moment to inspect it for signs of blockage.
A clogged fuel filter can prevent fuel from reaching the engine, causing it to stall. Check the fuel filter and replace it if necessary.
Check for Stale Fuel
If the gasoline in a lawnmower has been sitting for too long, it can become stale, gum up the carburetor, and cause the engine to stall. Drain the old fuel and replace it with fresh gasoline.
If possible, use an ethanol free gasoline because ethanol retains water and is more prone to leaving rust or a sticky film in the fuel system over time.
Test the Ignition Coil
Although insufficient spark from your mowers spark plug is not likely to be the cause of stalling, it should be tested as a contributing factor.
Familiarize yourself with an ignition coil resistance chart if you are testing an ignition coil with a multimeter for the first time. The chart shows you where to connect your multimeter and tells you what a good resistance range and reading are.
To remove an ignition coil for testing, simply remove the top cover of your lawnmower, locate the ignition coil, and remove the wire from the spark plug end, remove the 2 bolts holding it in place, and remove the kill wire from the ignition coil.
Tools required for ignition coil removal are some safety glasses, a 7 mm or 10 mm socket depending on mower model, and a little patience.
Check the Carburetor (Most Likely Cause)
Dirt, bad gas or a damaged diaphragm inside the carburetor is the most common cause of restricted fuel flow, and that is the most common cause of a stalling lawn mower engine.
I listed the steps above first because they take very little time to perform, and no new parts are required to rule them out as a cause. The carburetor is slightly more time-consuming to diagnose and repair.
Thankfully, a clogged carburetor can be diagnosed and cleaned easily, often without removing it from the engine. If you remove the carburetor, you may need to replace the carb gaskets before re-installing it.
Start by Inspecting The Carburetor Float Bowl
The carburetor float bowl on a lawn mower engine is located on the bottom of the carburetor. It is held in place by a single special bolt that has a small metering jet built into it.
Note: Some Honda carburetors have two bolts. Additionally, don’t lose the rubber o-ring sealing the bowl to the carburetor while you remove it.
Remove the bolt and the float bowl from the bottom of the carburetor. If you see any dirt in the float bowl, clean it out thoroughly using a soft brush or cloth and a carburetor safe cleaning agent.
Next inspect the bolt itself, the small fuel passage inside the bolt should also be cleaned to allow the carburetor to suction fuel from the float bowl. Carb cleaner in a spray can work well for these tasks.
Re-install the Float Bowl to See if The Problem is Resolved
With any luck, it was only dirt in the float bowl which was starving the engine of fuel by blocking the passageways. If the problem persists, a carburetor cleaning and rebuild may be needed.
If you wish to rebuild and clean your own carburetor, perhaps to save a little money, you can. You do, however, have other options, which I’ll detail below.
Cleaning and Rebuilding Your Lawnmower Carburetor (7 steps)
Before you get started cleaning or rebuilding a lawnmower carb yourself, I suggest you get an owner’s manual for your specific lawnmower model.
You can find owners manuals online and, frequently, on the manufacturer’s official website.
When you familiarize yourself with the steps you’ll need to perform, and assemble the necessary tools, the rebuilding and cleaning process will look something like this…
- Remove the carburetor from the lawnmower
- Disassemble the carburetor
- Clean the parts
- Replace worn or damaged parts
- Reassemble the carburetor
- Install the carburetor back on the lawnmower
- Adjust the carburetor
Alternative options to rebuilding a lawnmower carburetor
If you’d rather not remove and repair the carb yourself, you have two other options to consider. Both may be better for your lawnmower in the long run, depending on your proficiency in repairing small engines.
Alternative option #1 – Purchase a new carburetor – This option is appealing for the DIY minded individual who can remove and install the part themselves but would rather not spend the time cleaning and repairing it too.
This option is attractive if you can find a new carburetor at a reasonable price, which isn’t very difficult. A new carburetor for a used lawnmower is typically in the $25 – $60 price range on popular online marketplaces like Amazon and eBay.
Alternative option #2 – Bring the mower to a qualified local small engine mechanic. This option is likely the most expensive option and may take several days to get your mower running again, depending on how busy the shop is.
On the bright side, your mower will be fixed properly by an expert and should be problem free when you get it back from the shop.
Other Possible Reasons Your Mower Starts The Stalls
If you’ve exhausted all the steps above, there are other potential causes to look for in preventing the mower from stalling.
Although extremely rare, I’ve encountered these additional problems in the past a handful of times. In no particular order, here are other possible reasons for your mower starting and stalling quickly.
#1 – A damaged shear key on the flywheel. I once found the problem to be a heavily damaged shear key, which was engaging enough to allow the engine to start, but not intact enough to keep the engine running under load.
Replacing the shear key fixed this problem.
#2 – A loose kill wire on the engine coil. The wire was loose but was able to make sufficient contact to start the mower, however, it would lose contact when the engine vibrated forcefully enough.
Tightening the kill wire on the coil solved this issue.
#3 – A damaged mower blade. The engine would start, but violent vibrations caused by an unbalanced mower blade would make it stall very quickly. This problem is very obvious as the mower will almost dance when it starts due to vibrations.
Replacing the blade fixed the problem.
#4 – A loose throttle cable was also the cause in one instance. The cable itself was barely long enough to engage the engine, and the throttle lever had become sticky from dirt and oil accumulation.
The combination of these things was enough to allow starting the engine, but only momentarily. A good cleaning of the throttle lever, oiling of the throttle cable and tightened cable bolt fixed the problem.
Additional Troubleshooting Tips
If your mower is dirty, consider using a pressure washer to give it a good cleaning before you begin diagnosing your stalling engine. You don’t want dirt getting inside the filters or carburetor.
Additionally, keep an eye out for other problems as you look the mower over. Quite often it’s not just one thing that needs repair, it’s several. It’s best to prevent issues with good maintenance than to diagnose problems afterward.