If I had to pick the single most common — and most preventable — small engine problem, it would be stale fuel. I've rebuilt more carburetors because of old gas than every other cause combined. The fix is simple and costs under $10: fuel stabilizer.
What Is Fuel Stabilizer?
Fuel stabilizer is a chemical additive that slows the oxidation and evaporation of gasoline. Modern ethanol-blended fuel degrades faster than old fuel — within 30 days it can start forming gum and varnish that clog the microscopic passages in a small engine carburetor. Stabilizer extends the usable life of fuel from about 30 days to 12 months or more.
When to Use It
Use fuel stabilizer any time your equipment will sit unused for more than 30 days. Common scenarios:
- Mowers stored over winter
- Generators between power outages
- Chainsaws and trimmers between seasons
- Snow blowers stored over summer
- Pressure washers between uses
- Any equipment that gets used occasionally rather than weekly
How to Use Fuel Stabilizer Correctly
Step 1: Add Stabilizer to a Full Tank
Add the stabilizer to a full tank of fresh fuel, not a half-empty tank with old gas. The mixing ratio varies by brand — STA-BIL is 1 ounce per 2.5 gallons, while Sea Foam is 1 ounce per gallon. Read the label.
Step 2: Run the Engine
This is the step most people skip, and it's the most important. After adding stabilizer, run the engine for 5–10 minutes so treated fuel flows through the fuel line, fuel pump, and carburetor. If you only add stabilizer to the tank and store the equipment, the fuel already in the carburetor bowl will still go bad.
Step 3: Store the Equipment
After running the engine, store it normally. The treated fuel will remain stable for up to 12 months.
Common Mistakes to Avoid
Adding stabilizer to old fuel. Stabilizer prevents degradation — it doesn't reverse it. If your fuel is already 60 days old, adding stabilizer won't fix it. Drain the old fuel and start fresh.
Not running the engine. Stabilizer in the tank does nothing for the fuel in the carburetor. Always run the engine after adding it.
Using too much. Over-treating fuel won't damage the engine, but it's wasteful. Follow the label ratio.
Mixing brands. Stick with one brand. Different stabilizers use different chemistries and mixing them is unnecessary.
Which Stabilizer Should You Use?
I recommend two products:
- STA-BIL Storage — the standard for seasonal storage. Good for up to 12 months.
- Sea Foam — doubles as a fuel system cleaner. Good for both storage and ongoing use in equipment that gets used periodically.
For equipment that sits for very long periods (over a year), draining the fuel completely is more reliable than stabilizer.
Can You Use Stale Fuel?
If fuel has been sitting for more than 30 days without stabilizer, it's already degraded. You can sometimes get an engine to run on it, but it will run poorly and the varnish will continue to build up in the carburetor. The safest approach: dispose of old fuel at a local hazardous waste facility and start fresh.
The Bottom Line
Fuel stabilizer is the highest-return maintenance investment you can make for small engines. A $10 bottle prevents hundreds of dollars in carburetor repairs and hours of frustration. Treat your fuel, run the engine, and store with confidence.