Struggling to fire up that old mower, generator, or compact engine? Before you throw in the towel, let’s walk through the most common reasons why old small engines often refuse to start—and what you can do about them.
1. Stale or Degraded Fuel
Gasoline doesn’t age well. After sitting for over a month, it begins to oxidize and absorb moisture, turning into a thick, varnish-like substance. This bad fuel can clog the fuel system and lose its ability to combust properly, making engine startup a frustrating chore.
Fix: Drain the old fuel, clean the tank, and refill with fresh gasoline—ideally with a fuel stabilizer if storage is expected.
2. Clogged Carburetor
Fuel residue often gums up the carburetor when engines sit unused. Blocked jets or fuel passages prevent the engine from drawing the proper fuel-to-air mixture for ignition.
Fix: Remove and clean the carburetor with a quality carb cleaner. In severe cases, a rebuild kit or full replacement might be needed.
3. Faulty or Dirty Spark Plug
If the spark plug is fouled with carbon, has worn electrodes, or an incorrect spark gap, it may not produce a strong spark—or any spark at all. That’s a hard no-start right there.
Fix: Inspect, clean, or replace the spark plug. Make sure the gap is correctly set per manufacturer specs.
4. Dirty or Blocked Air Filter
Engines need clean air to run. A clogged air filter restricts airflow, upsetting the delicate air-fuel balance necessary for ignition.
Fix: Remove and inspect the air filter. Clean or replace it as needed for optimal airflow and performance.
5. Starter Mechanism Issues
If the engine won’t even crank, the starter system could be at fault. A jammed recoil rope, dead electric starter, or weakened spring starter (commonly used in diesel engines) can all prevent the engine from turning over.
Fix: Check for physical damage or wear in the starter components and repair or replace as needed. For spring starters, test tension and recoil strength.
6. Low or No Compression
Internal wear from age or poor maintenance can reduce engine compression. Worn piston rings, leaking gaskets, or damaged valves can all prevent the engine from compressing the air-fuel mixture properly—making ignition impossible.
Fix: Perform a compression test. If results are low, internal repairs like replacing rings or gaskets may be necessary.
