Annual Lawn Mower Service: Why and When
Service your petrol mower once a year — ideally in early spring before the mowing season begins. A clean carburettor, fresh oil and sharp blade make a dramatic difference to cut quality and engine longevity.
Tools You'll Need
- SAE 30 or 10W-30 engine oil (check your manual)
- Replacement air filter (foam or paper, model-specific)
- Replacement spark plug (NGK BR6HS or equivalent — check manual)
- Spark plug socket (usually 5/8" or 13/16")
- Blade removal tool or socket wrench
- Wire brush and carburettor cleaner spray
- Old oil drain tray
Step 1 — Change the Oil
- Run the mower for 5 minutes to warm the oil (drains more completely)
- Disconnect spark plug wire — essential safety step
- Tip the mower on its side (air filter side up to prevent oil entering it)
- Unscrew the oil drain plug or tilt to drain via the filler cap
- Let drain fully, then refill with correct grade to the min–max range
Step 2 — Replace the Air Filter
Remove the filter cover (usually one screw or clip). Foam filters: wash in soapy water, squeeze dry, re-oil lightly with engine oil and reinstall. Paper filters: tap gently to dislodge dust or replace with a new one.
Step 3 — Check the Spark Plug
Remove the plug with the spark plug socket. A good plug has a tan or light grey electrode. Replace if black and sooty (rich mixture), white and blistered (lean/overheating), or the electrode gap is too wide (check manual — typically 0.6–0.8 mm).
Step 4 — Sharpen or Replace the Blade
Tip the mower with the carburettor side up. Wedge a block of wood under the blade to stop it rotating. Remove the central bolt (usually 16mm or 5/8"). Sharpen with a file or bench grinder, maintaining the original 30–45° bevel angle. Balance the blade on a nail — if one side dips, grind more from that side. An unbalanced blade causes vibration that damages bearings.
Step 5 — Clean the Underside
Dry grass compacted on the underside restricts airflow and clogs the discharge chute. Scrape clean with a wooden or plastic scraper (never metal — it damages the aluminium deck). Spray with a hose on low pressure.
Step 6 — Test Start
Reconnect the spark plug wire. Fill with fresh petrol. Start and check for smooth idle, no smoke, and clean cut on a test pass.
FAQ
How do I know when my blade needs sharpening?
If the cut grass has ragged, torn or brown tips rather than clean cuts, the blade is dull. Dull blades also stress the grass and invite disease.
