Quick answer
If your lawn mower won’t start after winter storage, the most common culprits are stale fuel degrading in the system, a carburetor clogged with fuel varnish, and a spark plug fouled by old fuel residue. The fix involves draining the old gas, cleaning or rebuilding the carburetor, and installing a fresh spark plug.
Few things are as frustrating as the first mow of the season being stalled by a mower that refuses to start. When a lawn mower won’t start after winter storage, the root cause is almost always related to the fuel system. Gasoline left sitting for months breaks down, forming gummy deposits that clog the small passages in the carburetor and coat the spark plug. This guide focuses on the specific steps to diagnose and fix these post-storage issues, helping you get your mower running smoothly for the season ahead. For a broader look at starting problems, see our article on common causes for a mower that won’t start.
Why Winter Storage Causes Starting Problems
During months of inactivity, the volatile compounds in gasoline evaporate, leaving behind a thicker, less combustible fuel that can turn into a sticky varnish. This stale fuel can’t properly vaporize for combustion and leaves residues that block the tiny jets and ports in your carburetor. Additionally, moisture from condensation can mix with the fuel, leading to corrosion and further clogging. The spark plug can also become fouled with carbon deposits from incomplete combustion of the old fuel during your last mow of the previous season. Addressing these three interconnected issues—fuel, carburetor, and spark—is the key to a successful spring start.
Safety
Before beginning any work, always disconnect the spark plug wire to prevent accidental starting. Work in a well-ventilated area away from sparks or open flames, as you’ll be handling gasoline. Wear safety glasses and gloves. If you are unsure about any step, consult your mower’s manual or contact a qualified small engine repair professional.
Step 1: Diagnose the Fuel System
Your first step is to determine if stale fuel is the problem. Start by checking the fuel tank. If you left gas in it over the winter, smell it. Fresh gasoline has a distinctive, sharp odor, while stale fuel often smells sour or like varnish. Visually, old fuel may appear darker than normal.
How to Drain Old Gasoline
If the fuel is old, you must drain the entire system. Use a hand pump or a siphon to remove all the old gasoline from the tank into an approved fuel container. Never siphon by mouth. After draining the tank, you also need to drain the carburetor bowl. Most carburetors have a small drain screw at the bottom of the bowl; place a container underneath, loosen the screw, and let the old fuel drain out. With the system empty, you can add fresh, stabilized gasoline. Using a fuel stabilizer in your last tank before storage is the best way to prevent this issue next year. For more seasonal care tips, visit our lawn mower maintenance hub.
Step 2: Clean or Rebuild the Carburetor
If fresh fuel doesn’t solve the problem, the carburetor’s internal passages are likely clogged with varnish from the old gas. This is a very common reason a lawn mower won’t start after winter.
What to look for
- Disassembly: Carefully remove the carburetor from the engine, noting the position of all linkages and gaskets.
- Visual Inspection: Look for gummed-up fuel in the bowl and clogged jets. The main jet and pilot jet are small brass pieces with tiny holes.
- Cleaning: Soak metal carburetor parts in a commercial carburetor cleaner, then use compressed air to blow through all passages. Never use a wire to clean jets, as it can enlarge the holes.
- Rebuild Kit: For heavily varnished carburetors, a rebuild kit with new gaskets, seals, and sometimes a new float needle is often necessary.
After cleaning and reassembling the carburetor, reinstall it on the engine, reconnect the fuel line, and try starting the mower again. A clean carburetor ensures the correct air-fuel mixture reaches the engine.
Step 3: Check and Replace the Spark Plug
A spark plug fouled by carbon deposits or fuel residue may not produce a strong enough spark to ignite the fuel mixture. This is a quick and inexpensive check.
- Remove the spark plug wire boot.
- Use a spark plug socket to carefully unscrew the plug from the engine.
- Inspect the plug. The electrode should be a light tan or gray color. If it’s black, wet with fuel, or coated with heavy deposits, it needs to be replaced.
- Check the gap on a new plug (or the old one if it looks clean) with a gap tool, setting it to the specification in your mower’s manual.
- Thread the new plug in by hand to avoid cross-threading, then tighten it with the socket.
- Reconnect the spark plug wire securely.
A fresh spark plug provides the reliable ignition needed for starting. If you’re working through multiple issues, our lawn mower troubleshooting checklist can help you stay organized.
When Basic Fixes Aren’t Enough
If you’ve addressed the fuel, carburetor, and spark plug and the mower still won’t start, other issues may be at play. Compression and ignition problems can also mimic fuel system failures. Ensure the flywheel key isn’t sheared, which would throw off ignition timing. Verify that the engine has sufficient compression by pulling the starter cord—it should have a strong, consistent resistance. Also, check that the fuel shut-off valve (if equipped) is in the “On” position and that the air filter isn’t completely clogged, as a starved engine can’t start. For problems that manifest during mowing, like stalling, see our guide on why a mower engine stalls while mowing.
| Symptom | Likely Cause | Primary Fix |
|---|---|---|
| Engine turns over but won’t fire | Stale fuel in carburetor | Drain fuel, clean carburetor |
| Mower starts briefly then dies | Clogged carburetor jets | Clean or rebuild carburetor |
| No spark at the plug | Fouled or dead spark plug | Replace spark plug |
| Fuel smell, wet spark plug | Flooded engine from old gas | Dry plug, drain system, use fresh gas |
Frequently Asked Questions
Can I use old gas if I add fresh gas to it?
We do not recommend mixing old and new gasoline. The stale fuel has already degraded and lost its volatility, and diluting it with fresh gas won’t restore its properties or dissolve the varnish it has created. It’s best to completely drain and dispose of old gasoline properly and start with a full tank of fresh fuel.
How do I prevent this from happening next winter?
The single most effective prevention is to either run the engine until it stalls from fuel starvation after adding a fuel stabilizer to the last tank, or to manually drain the entire fuel system (tank, fuel line, and carburetor bowl) before storing the mower. Adding a stabilizer to a full tank and running the engine for a few minutes to circulate it through the carburetor is the most common method for gasoline engines.
My electric mower won’t start after winter. What should I do?
For battery-electric mowers, a lawn mower won’t start after winter usually points to a battery issue. The battery may have self-discharged below its safe operating voltage during storage. First, ensure it is properly seated and connected. Then, try charging it fully with the manufacturer’s charger. If it doesn’t hold a charge, the battery may need to be replaced. For a detailed walkthrough, see our article on troubleshooting an electric mower battery that won’t charge.
What if cleaning the carburetor doesn’t work?
If a thorough cleaning doesn’t resolve the issue, the carburetor may be worn or damaged beyond simple cleaning. Internal seals can dry out and leak, and the float valve may not seat properly. In this case, using a complete carburetor rebuild kit or replacing the entire carburetor assembly is often the most reliable solution. If you’re not comfortable with this level of repair, a small engine shop can typically handle it quickly.
References
- Manufacturer manuals and specification sheets: For model-specific disassembly steps, torque specs, and part numbers.
- U.S. Consumer Product Safety Commission: Lawn mower safety guidance and recall information at CPSC Recalls.
- ANSI/OPEI B71 safety standards: Standard safety specifications for powered lawn equipment, provided by OPEI.
