Your lawn mower, ATV, or generator won't start? Worn out spark plugs are often the culprit. Replace them yourself in 5 minutes for $5-15. Works on all 4-stroke engines.
Small engine spark plugs are larger and more robust than car spark plugs. They must match your specific engine model. Common types include Champion, NGK, and Briggs & Stratton brands. The spark plug ignites the fuel-air mixture inside the engine cylinder. When worn, they cause hard starting, rough running, or complete failure to start.
Check your owner's manual for the exact spark plug model and gap specification. The gap is the distance between the center and ground electrodes. Most small engines use 0.028"-0.035" gap. If you can't find your manual, search "[Brand] [Model] spark plug" online.
Look for a thick wire or cap on top of the engine. This connects to the spark plug below. The spark plug is threaded into the cylinder head. Gently grasp the wire/cap (not the wire itself) and twist and pull to disconnect it from the spark plug.
Insert the spark plug socket onto the spark plug. Attach the ratchet wrench to the socket. Turn counterclockwise (righty-loosey, lefty-tighty in reverse). The plug should come free in 2-3 turns. Gently pull the socket and plug out. Examine it — fouled plugs are black, wet, or crusty.
New plugs usually come pre-gapped, but verify. Use a spark plug gap tool or feeler gauge. Insert the gauge between the center and ground electrode. It should slide in snugly. If the gap is too wide or narrow, gently adjust the ground electrode with a gap tool until it's correct.
Insert the new spark plug into the spark plug socket. Guide it into the cylinder head hole. Turn the wrench clockwise slowly. The plug should thread in smoothly. Tighten firmly but not excessively — you'll hear/feel when it seats. Most small engine plugs need about 20 ft-lbs of torque (snug, not gorilla-tight).
Take the spark plug wire/cap and firmly push it down onto the new spark plug. You should feel it seat with a slight snap. Pull gently to confirm it's on tight. If it comes off easily, push it down again until it's secure.
With everything reconnected, start the engine. It should fire up immediately or within a few pulls. Let it idle for 30 seconds. The engine should run smoothly without stuttering or black smoke. If it does, you've fixed it!
Wrong Spark Plug Type: Always double-check the model number. A mismatched plug can damage the engine.
Incorrect Gap: The gap must be precise. Too wide or too narrow causes starting problems or misfires.
Over-tightening: Don't crank down on the plug. Just snug it in. Over-tightening strips the cylinder head threads.
Not Seating the Wire: Make sure the spark plug wire is fully connected. A loose wire causes weak spark or no spark.
Replace every 100-200 hours: Regular replacement prevents starting problems and ensures peak engine performance.
Store spare plugs: Keep an extra spark plug on hand. When your mower won't start mid-season, a fresh plug might be all you need.
Got stuck during the spark plug replacement? Contact us.
Get Help Back to Guides