How do I service a petrol lawnmower myself?
The four jobs that make up an annual service — oil, air filter, spark plug and blade — and the order and details that make each worth doing.
- Difficulty
- intermediate
- Time
- 1 hr 30 min
- Read
- 5 min
- Safety
- warning
Short answer
Change the oil while the engine is warm, clean or replace the air filter, fit a new spark plug set to the right gap, and sharpen and balance the blade. Those four cover almost everything a workshop does on an annual service, and they take about an hour and a half.
A small four-stroke engine is a simple machine and its annual service is genuinely within reach. What separates a good home service from a poor one is doing the whole set rather than only the easy parts, and paying attention to two details — the oil grade and the blade balance — that get skipped.
Safety
What you'll need
- Engine oil of the grade in the handbook
- A drain container and a funnel
- A new air filter, or foam filter oil
- A new spark plug and a plug spanner
- A file or a bench grinder, and a blade balancer or a nail
Step by step
- Run the engine for a few minutes first.Warm oil drains far more completely and carries the suspended dirt with it. Then switch off and pull the plug lead.
- Check the handbook for the oil grade and quantity.Small engines are specific about this, and quantities are small — often under 600ml. Overfilling causes smoking and oil in the air filter, and is as bad as underfilling.
- Drain the old oil.Either through the drain plug if there is one, or by tipping the machine with the filler uppermost into a container. Do not tip a mower with the carburettor side down.
- Refill slowly and check on level ground.Add most of it, then check the dipstick or the fill line, then top up in small amounts. It is very easy to overfill a small engine because the total quantity is so small.
- Dispose of the old oil properly.Household waste sites take engine oil. It must never go down a drain, onto the ground or into a bin — it is a serious pollutant and it is an offence.
- Deal with the air filter.Foam: wash in warm soapy water, squeeze rather than wring, dry completely, then work a little clean engine oil through it and squeeze out the excess. Paper: replace it. A paper filter cannot be washed and blowing it out with compressed air damages the element.
- Fit a new spark plug.Check the gap against the handbook figure with a feeler gauge or a gapping tool — most small engines want around 0.7 to 0.8mm. Tighten by hand until it seats, then the amount the plug manufacturer specifies, usually a further half turn. Over-tightening strips an aluminium head.
- Remove the blade with the plug lead off.Wedge a block of wood between the blade and the deck to stop it turning. Note which way up it fits — the wings or lift edges face upwards, towards the deck — and photograph it before removal.
- Sharpen along the existing bevel.A file or a bench grinder, following the angle that is already there, taking the same amount off each end. Aim for an edge you would not want to run a thumb along, not a razor edge — a mower blade cuts by impact and a very fine edge just folds over.
- Balance the blade before refitting.Hang it on a nail through the centre hole, or use a balancer. If one end drops, take a little more metal off that end. An unbalanced blade shakes the engine, the crankshaft and the whole machine and shortens their life considerably.
- Refit the blade the right way up and to the correct torque.The bolt is usually specified quite tightly. A blade that comes loose in use is dangerous.
- Clean the deck and the cooling fins.Scrape the deck, brush out the fins and the recoil housing. Cooling and airflow are the two things a small engine cannot do without.
- Reconnect the plug lead and test.Outdoors. Run it for a few minutes, check for leaks around the plug and the oil filler, and listen for vibration that would suggest the blade is out of balance.
Tips
- Do the whole service at the end of the season, not the start. The machine is warm, you are outside, and spring is when parts are out of stock.
- Buy the plug, the filter and a blade at the same time. The postage costs more than the difference.
- Keep the handbook, or download a PDF and save it. The oil grade, oil quantity and plug gap are all in it and all matter.
Common mistakes
- Refitting a sharpened blade without balancing it — Metal removed unevenly leaves one end heavier. At mowing speed that produces a vibration which is transmitted straight into the crankshaft and bearings, and it destroys engines.
- Overfilling the oil — Small engine sumps hold very little, so a splash too much is a large percentage. The excess is thrown around, forced past seals and into the air filter, and the machine smokes and runs badly.
If it doesn't work
Smokes badly after the service
Cause: Overfilled oil, or oil in the air filter from tipping — Fix: Check the level and drain to the mark; clean or replace the air filter.
Vibrates more than before
Cause: Unbalanced or incorrectly fitted blade — Fix: Remove, balance, and refit the right way up.
Hard to start after fitting a new plug
Cause: Wrong plug or wrong gap — Fix: Check the plug part number against the handbook and set the gap with a feeler gauge.
Grass cut looks torn and brown at the tips
Cause: Blade sharpened at the wrong angle or still blunt — Fix: Re-sharpen along the original bevel; a torn tip is the classic sign of a blunt blade.
Questions people ask
How often does a mower need servicing?
Once a year for a domestic machine, at the end of the season. Air filter and oil are the two that matter most; the blade should be checked several times a season if you mow a large or stony lawn.
Can I use car engine oil?
Check the handbook. Many small four-strokes specify a straight SAE 30 or a 10W-30, and while some car oils are suitable, the grade and the specification matter more than the label. Two-stroke engines take no sump oil at all — the oil goes in the fuel.