If a petrol mower drags on the pull cord, fires then dies, runs rough or chews through more fuel than it used to, the spark plug is the cheapest part to suspect first. A new plug costs only a few pounds, the job takes about ten minutes, and the only tools you need are a 13/16 inch (21mm) plug socket and a feeler gauge. Done once a season, it is the difference between a first-pull start in June and a frustrating ten minutes of yanking the cord while the grass keeps growing. Here is how to change one properly, set the gap, and read the old plug for clues about the engine.
Why the spark plug fails, and the warning signs
The spark plug sits at the top of the cylinder and does one job: it produces the spark that ignites the fuel and air mixture, dozens of times a second, every time the engine turns. The tip lives in a brutal place. Each spark erodes a tiny amount of metal from the electrodes, so over hundreds of hours the gap between them widens. Combustion also leaves deposits of carbon, oil and fuel additives baked onto the tip. A worn or fouled plug produces a weaker, less reliable spark, and a weak spark is the root of most early-summer starting trouble.
The signs are easy to spot once you know them. Hard starting, where the engine needs many pulls to catch, is the classic symptom. So is an engine that fires and then stalls, runs unevenly or surges, or one that misfires under load when you push into thicker grass. Rising fuel use and a smell of unburnt petrol point the same way, because a poor spark leaves fuel unburned. Most small-engine makers, including Briggs and Stratton and Honda, recommend a fresh plug roughly once a year or every 25 hours of running, which for a typical garden lawn lines up neatly with a start-of-season service. A plug is cheap insurance, so when in doubt, change it.
Choosing the right plug and the tools for the job
The safest way to buy the correct plug is to read the code printed on the old one and match it, or to check the engine manual. Two plugs cover a huge share of garden mowers. The NGK BPR6ES is a common fit, with a specified gap around 0.7 to 0.9mm (0.028 to 0.035 inches). The Champion RJ19LM is the other workhorse, with a gap of roughly 0.7 to 0.8mm (0.029 to 0.033 inches). Both have a 14mm thread and a 13/16 inch hex, which is why one socket fits most mowers. A genuine plug costs around 4 to 8 pounds (about 5 to 10 dollars) at Screwfix, B&Q, Home Depot, Amazon or any mower dealer. Avoid the temptation to fit whatever is in the drawer: the wrong heat range or reach can foul quickly or, worse, protrude into the cylinder and strike the piston.
For tools you need a spark plug socket, which is a deep 13/16 inch socket with a rubber insert that grips the plug so it does not drop or crack, at around 5 to 8 pounds (about 6 to 10 dollars). A feeler gauge to set the gap costs about 4 to 6 pounds (about 5 to 8 dollars), and a small wire brush helps if you want to inspect rather than replace. A ratchet or a short extension bar makes turning easier. That is the whole kit, and it will last for years across every petrol engine you own.
Changing the plug step by step
Start with safety. Let the engine cool completely, because the plug and cylinder head get hot enough to burn. Pull the rubber spark plug cap off the end of the plug so the engine cannot fire while your hands are near the blade, and if you want to be doubly safe, tip the mower with the air filter and carburettor side facing up so fuel and oil cannot run into them. Never tip it the other way, or oil floods the filter and you swap one problem for another.
Brush or wipe away any dirt around the base of the plug so nothing falls into the cylinder when you remove it. Fit the plug socket squarely over the plug and turn anticlockwise to unscrew it. Lift the old plug out and keep it to read in a moment. Take the new plug and set the gap before fitting: slide the correct feeler blade between the centre and side electrodes. It should pass with a light drag. If it is loose, gently close the side electrode a fraction; if it will not enter, ease the side electrode open. Bend only the side electrode, never the centre one, and only in small movements.
Screw the new plug in by hand first, turning it several full turns with your fingers. This single step prevents cross-threading, which can ruin the soft aluminium thread in the cylinder head and turn a five pound job into a costly repair. Once it is finger tight against its seat, nip it up with the socket. For a plug with a gasket, the rule of thumb is finger tight plus about half a turn with the wrench, which seats the gasket without overtightening. If you have a torque wrench, aim for roughly 25 to 30 newton metres (around 18 to 22 pound feet) on an aluminium head. Refit the spark plug cap firmly until it clicks home, set the mower back level, and it is ready to start.
Can you clean a plug instead of replacing it
You can clean a plug to get you out of trouble, but it is rarely worth the bother given how cheap a new one is. If a plug is lightly fouled with dry carbon and you are mid-mow with no spare, remove it, brush the deposits off the tip and electrodes with a small wire brush, wipe it clean, reset the gap with the feeler gauge, and refit it. That will often restore a usable spark for the rest of the job. Avoid sandblasting or filing the electrodes, as both leave the surface rough and shorten the plug’s life.
What cleaning cannot fix is a worn plug. Once the electrodes have eroded to the point where the gap keeps drifting wide, or the centre electrode has rounded off from years of sparking, no amount of brushing brings back a strong, consistent spark. At that stage a clean plug is a temporary patch, not a repair. Treat cleaning as the roadside fix and a fresh plug as the proper service, and keep a spare in the shed so you are never stuck. A fresh plug also helps the engine burn fuel completely, which trims fuel use and cuts the unburnt fumes a tired plug leaves behind.
Read the old plug before you bin it
The old plug is a free diagnostic report on your engine, so glance at the tip before you throw it away. A light tan or grey-brown colour on the tip is what you want, and it tells you the engine is running cleanly and the plug heat range is correct. Dry, black, sooty deposits point to a rich mixture or a clogged air filter starving the engine of air, so check and clean or replace the filter next. Wet, oily black deposits suggest oil is reaching the combustion chamber, often from overfilling the sump or tipping the mower the wrong way, and worth watching if it returns. A clean white or blistered tip can mean the engine is running too hot or the plug heat range is wrong.
Getting it wrong has real consequences. Overtightening strips the thread and can crack the ceramic insulator, which then has to be drilled out by a dealer. Leaving a plug loose lets compression escape, so the engine loses power and the plug overheats. Setting the gap too wide makes the spark struggle to jump, causing misfires, while too narrow a gap gives a weak spark that fouls quickly. Fit the right plug, set the gap, start it by hand and finish it snug, and your mower should fire on the first or second pull all summer. Keep the old socket and feeler gauge with your mower kit, alongside the spare oil and air filter, and a seasonal plug change becomes a five minute habit rather than a breakdown waiting to happen.
