The spark plug is one of the cheapest parts on your small engine, but it's also one of the most common causes of starting problems, rough running, and power loss. Here's how to know when to replace it — and how to read what your old plug is telling you.
How Often Should You Replace a Spark Plug?
The general rule for small engines:
- Replace every season if you use the equipment regularly (weekly mowing, frequent generator use)
- Replace every 100 hours of operation for heavy-use equipment
- Replace immediately if the engine won't start, runs rough, or lacks power and you haven't replaced the plug in over a year
A new spark plug costs $3–6. If you're troubleshooting a running problem and the plug is old, replace it first — it's the cheapest and easiest thing to try.
Signs You Need a New Spark Plug
- Hard starting — the engine cranks but takes many pulls to start, or won't start at all
- Rough idle or surging — the engine revs up and down unpredictably
- Loss of power — the mower bogs down in tall grass, the generator can't handle its rated load
- Increased fuel consumption — the engine runs rich because the spark is weak
- Visible damage — cracked porcelain, eroded electrode, heavy carbon buildup
How to Inspect a Spark Plug
Step 1: Remove the Plug
Disconnect the spark plug boot. Use a spark plug socket and ratchet to remove the plug. Keep the area around the plug clean so no debris falls into the cylinder.
Step 2: Read the Plug
The condition of the plug tells you what's happening inside the engine:
Normal: Light tan or gray coating on the insulator tip. The electrode is worn but not eroded. This plug is still serviceable but may need replacement if it's old.
Dry and black (sooty): The engine is running rich — too much fuel, not enough air. Causes: dirty air filter, choke stuck closed, carburetor needs adjustment. Replace the plug, but also fix the underlying cause.
Wet and black (oily): Oil is entering the combustion chamber. Causes: worn piston rings, worn cylinder, blown head gasket, or overfilled crankcase. This is a more serious problem — replacing the plug won't fix it.
White or blistered: The engine is running lean — too much air, not enough fuel. Causes: carburetor needs adjustment, intake leak, or clogged fuel passages. This can cause engine damage if not corrected.
Electrode eroded or melted: The plug is worn out or the engine is overheating. Replace the plug and check the cooling fins for debris.
Step 3: Check the Gap
The gap between the center and side electrode must match the spec in your owner's manual — typically 0.030" for most small engines. Use a feeler gauge to check. If the gap is too wide, the spark is weak. If it's too narrow, the spark can't jump the gap.
You can gap a new plug by gently tapping the side electrode on a flat surface, but it's easier to buy a plug that's pre-gapped for your engine.
How to Replace a Spark Plug
- Gap the new plug to the spec in your manual
- Thread it in by hand to avoid cross-threading — never force it
- Tighten with a socket — 1/2 to 2/3 turn after the gasket contacts the seat (or 1/16 turn if no gasket)
- Reconnect the boot — push it on until it clicks
Don't overtighten — you can crack the porcelain or damage the threads in the cylinder head.
What Spark Plug Does My Engine Need?
Check your owner's manual for the correct part number. Common small engine spark plugs:
- Champion RC12YC — Briggs & Stratton, many riding mowers
- Champion RJ19LM — Briggs & Stratton push mowers
- NGK BPR5ES — Honda GX series engines
- NGK BPMR7A — many 2-stroke engines (chainsaws, trimmers)
Using the wrong plug can cause poor performance or engine damage. When in doubt, use the exact part number from the manual.
The Bottom Line
If your small engine won't start or runs rough and the spark plug is more than a year old, replace it. It's a $5 part that solves a surprising number of problems. Read the old plug before you throw it away — it tells you what's happening inside the engine.