Has your lawn mower been sitting all winter, or even just a few weeks, and now won’t start? This is one of the most common mower problems. After storage—especially in the off-season—fuel and engine issues are far more likely. The good news? Most of these problems are simple to fix, and a little know-how will have your mower running again in no time. This in-depth guide covers every cause, solution, and maintenance tip so you can get back to mowing quickly.
Why Won’t a Lawn Mower Start After Sitting?
- Old or stale gasoline in the tank or carburetor
- Gummed-up carburetor jets or stuck float/needle valve
- Dirty, fouled, or corroded spark plug
- Clogged or dirty air filter
- Low oil or old, thick oil (especially in cold weather)
- Dead or weak battery (for electric start or cordless mowers)
- Blocked fuel lines or clogged fuel filter
- Water in the fuel from condensation
- Rodent or insect nests in the air intake or exhaust
- Seized or stuck engine/blade (after sitting for years)
1. Start with Fresh Gasoline
- Always drain and replace old gas before trying to start a mower after storage.
- Stale fuel turns to varnish and gums up the carburetor, preventing starting.
- Use regular unleaded gas (E10 or less ethanol if possible). Add a stabilizer for better storage next time.
- If you suspect water in the tank (common after condensation), drain it fully and clean the tank before refueling.
2. Inspect and Clean the Carburetor
- Stale fuel is the #1 cause of carburetor problems after sitting.
- Remove the air filter and spray carburetor cleaner into the intake. Try to start—if it fires briefly, you likely have a gummed-up carb.
- Remove the carburetor bowl and clean jets, float, and needle with carburetor cleaner and compressed air.
- If the carb is heavily varnished, rebuild or replace it. Most mowers use inexpensive carbs that are easy to swap.
- Reinstall all gaskets and check for leaks before starting again.
3. Check and Replace the Spark Plug
- Remove the spark plug and inspect for black soot, rust, corrosion, or cracks.
- Clean lightly with a wire brush, or replace with a new plug if fouled or old.
- Ensure the gap matches your mower’s specs (usually 0.028–0.030 inches).
- Reconnect the spark plug wire firmly after reinstalling.
4. Examine and Clean the Air Filter
- A clogged air filter prevents proper airflow and can stop a mower from starting after sitting.
- Remove and inspect the filter. Clean foam filters with warm, soapy water, then dry and oil lightly.
- Replace paper filters if dirty or dark.
- Try starting the mower briefly without the filter (just for a test). If it runs, install a fresh filter.
5. Change the Oil
- Old oil becomes thick, collects water, and can prevent easy starting—especially after storage.
- Drain and refill with fresh SAE 30 or the oil grade specified for your mower.
- Always check oil level before the first start each spring.
- If oil appears milky, there may be water in the crankcase—change immediately.
6. Check the Battery (Electric Start & Cordless Mowers)
- Batteries lose charge during storage, especially in cold weather.
- Charge mower or 40V/18V batteries fully before attempting to start.
- Inspect terminals for corrosion and clean as needed. Replace batteries that fail to hold a charge.
- For corded mowers, check the power outlet, extension cord, and reset any GFCI breakers.
7. Inspect Fuel Lines and Filter
- Rubber lines can dry out, crack, or collapse after sitting.
- Check all hoses for cracks, brittleness, or blockages. Replace if needed.
- Replace the fuel filter if dirty or slow-flowing.
8. Look for Mechanical Obstructions
- Rodents or insects sometimes nest in mowers during storage—check air intake, muffler, and around the engine for debris.
- Remove the spark plug and try turning the blade by hand (wear gloves). It should spin freely. If stuck, inspect for jams or possible internal engine damage.
9. Reset Safety Switches and Controls
- Ensure all safety levers, handles, and switches are in the proper start/run position.
- On riding mowers, check seat, brake, and PTO switches. On cordless mowers, check battery seating, safety key, and handle position.
- Release and re-engage all controls to reset.
10. Flooded Engine? Here’s What to Do
- If you smell gas or the plug is wet, your engine may be flooded after multiple start attempts.
- Remove the spark plug, let it air dry, and pull the starter cord several times to clear excess fuel from the cylinder.
- Wait 10–15 minutes before reinstalling the plug and trying again (without priming or using the choke).
Preventive Tips: Store Your Mower for Easy Starting Next Time
- Run the engine dry or add fuel stabilizer before winter storage.
- Change the oil, clean the carburetor, and replace the air filter before storage.
- Store in a dry, sheltered spot to prevent moisture, rust, and pest damage.
- Remove batteries and store indoors for cordless or electric models.
Frequently Asked Questions (FAQ)
Why won’t my lawn mower start after winter?
Most commonly, stale fuel or a gummed-up carburetor is to blame. Always replace old gas and clean the carburetor after storage.
How do I clean a carburetor that sat with old gas?
Remove the carburetor, disassemble, and soak metal parts in carburetor cleaner. Clean all jets and passages, then reassemble with fresh gaskets.
Should I change the spark plug every spring?
It’s a good idea for easy starting. At a minimum, clean and inspect it each season.
My battery mower won’t turn on after sitting—what do I do?
Charge the battery fully, clean terminals, and check for error lights. Replace the battery if it won’t hold a charge.
Is it safe to use starting fluid after storage?
Yes, in small amounts—just a quick spray in the carburetor can help diagnose fuel issues. Avoid overuse, as it can wash oil from engine parts.
Conclusion: Get Your Mower Running After Storage
A mower that won’t start after sitting is usually an easy fix—replace old fuel, clean the carburetor and spark plug, and check the battery or filters. With regular preventive maintenance, you can avoid most storage-related issues and enjoy reliable starts all season long. Don’t let a stubborn engine keep you from a tidy lawn—follow these steps, and your mower will roar back to life in no time!
Ready to mow? Give your stored mower a little TLC, and you’ll be back in action faster than you think!