A mower that starts first pull in spring, then becomes hard work a few weeks later, often has a simple cause: the plug is no longer firing as it should. Knowing when to replace spark plug is basic preventative maintenance for petrol-powered garden, workshop and site equipment. It costs little, takes little time and can prevent a machine from being sidelined when there is a job to finish.
Spark plugs are a service item. They wear gradually, and their condition can also reveal problems with fuel, oil, air intake or engine adjustment. Replacing one at the right point is sensible. Replacing it without checking why it failed can mean the same fault returns.
When to replace spark plug: the practical answer
For many small petrol engines, checking the spark plug at least once a season or every 50 to 100 running hours is a sensible working routine. Replacement intervals vary by engine and plug type, however, so the engine handbook remains the proper reference. A basic nickel or copper-core plug generally needs changing sooner than a long-life platinum or iridium plug.
In real workshop use, replace the plug if it is badly fouled, the electrode is worn or damaged, the ceramic insulator is cracked, or the specified electrode gap can no longer be set accurately. If a machine has an unknown service history, fitting the correct new plug is often worthwhile after checking the rest of the ignition and fuel system.
For equipment that earns its keep - generators, water pumps, pressure washers, plate compactors, hedge cutters and ride-on mowers - do not wait for a complete no-start before carrying out the check. A plug that is only just firing can make an engine difficult to start, uneven under load and heavier on fuel.
Signs the plug is past its best
A weak spark can look much like a fuel problem. The engine may crank normally but take several pulls or extended starter-motor use before it fires. It may start with choke, then hesitate or stall as the choke is opened. Under load, it can misfire, lose power, hunt at a steady speed or produce an occasional pop from the exhaust.
These symptoms do not automatically condemn the plug. Stale petrol, water in the fuel, a blocked fuel filter, a dirty carburettor, a restricted air filter or a failing ignition coil can produce similar behaviour. The spark plug is still a sensible first inspection because it is accessible and gives useful evidence.
Look at both the centre electrode and the earth electrode. Their edges should not be heavily rounded away. Excessive wear increases the gap and makes the spark harder to produce, especially when the engine is working hard or the battery is weak. A visibly worn electrode is reason enough to renew the plug rather than trying to get another season from it.
What plug deposits are telling you
Remove the plug only when the engine is cool, then inspect it in good light. The condition of the firing end matters more than whether the outer body looks old.
A light tan or grey deposit is normally a healthy sign. A dry, sooty black coating commonly points to an over-rich mixture, excessive choke use, a blocked air filter or repeated short runs where the engine never reaches full operating temperature. Clean the plug if it is otherwise serviceable, but rectify the cause. A new plug will also soot up if the engine is still running rich.
A wet black plug may be fuel-fouled after repeated unsuccessful starting attempts, or oil-fouled if oil is entering the combustion chamber. On a two-stroke machine, confirm that the correct fuel-to-oil ratio is being used. Too much two-stroke oil, poor-quality oil or an engine wear issue can foul plugs quickly.
A white, blistered insulator or damaged electrode can suggest overheating or a mixture that is too lean. This is not a condition to ignore. Check cooling fins, air leaks, fuel supply and the correct plug specification before putting the machine back into regular service. A plug with a cracked insulator, melted electrode or severe deposits should be replaced, not cleaned and reused.
Do not guess the replacement plug
The correct replacement is more than a plug that happens to fit the thread. Thread diameter, reach, heat range, terminal type and electrode design must match the engine specification. A plug with the wrong reach can sit too far from the combustion chamber or project too far into it. Either situation can affect running and, in the worst case, cause mechanical damage.
Heat range is equally important. A plug that runs too cold can foul more readily; one that runs too hot may contribute to overheating and pre-ignition. Use the manufacturer’s stated plug code or an established cross-reference for the exact engine model. This matters on Honda, Stihl, Husqvarna, Lister and other small-engine applications where models within one product range can use different specifications.
Resistor plugs are also not interchangeable in every application. Many modern engines and electronic ignition systems specify a resistor type to reduce electrical interference. If the old plug has an R in the stated code, do not replace it with a non-resistor version unless the engine manufacturer specifically allows it.
Fitting a spark plug without causing a bigger repair
Before removal, brush or blow loose dirt away from the plug recess. Grit dropped into the cylinder is an avoidable problem, particularly on machinery used around soil, dust and demolition debris. Disconnect the plug cap by gripping the boot, not pulling on the lead.
Thread the new plug in by hand first. It should screw in smoothly for several turns. If it does not, stop and realign it. Cross-threading a plug into an aluminium cylinder head can turn a routine service job into a thread-repair job.
Set the electrode gap to the engine specification using a proper feeler gauge where the plug type permits adjustment. Some fine-wire plugs should not be aggressively bent or gapped, so follow the plug maker’s instructions. Tighten with a torque wrench where possible, using the engine or plug manufacturer’s torque figure. Overtightening can damage threads or the plug; undertightening can cause poor heat transfer and compression leakage.
Refit the cap securely, then start the engine and allow it to warm up. Check that it idles cleanly, responds to throttle and runs properly under normal load. If the problem remains, move on to the fuel, air and ignition checks rather than fitting plug after plug.
Service intervals depend on how the machine is used
A domestic lawn mower used for an hour a week in dry weather will not age its spark plug in the same way as a generator operating long shifts on site. Dusty conditions, frequent idling, short runs, cold starts and poor fuel storage all shorten plug life. Two-stroke equipment also places different demands on a plug than a four-stroke engine.
For machines used professionally, include the plug in a planned service routine alongside oil changes, air-filter checks, fuel-filter replacement and cooling-fin cleaning. Keeping the correct spare plug in the workshop or service vehicle is also good practice. It is a low-cost item that can save a wasted call-out or a delayed day’s work.
A quick check before you blame the plug
If a new correctly specified plug does not improve starting or running, check the basics. Make sure there is fresh petrol, the fuel tap is open where fitted, the air filter is not blocked and the stop switch is not holding the ignition off. Inspect the plug cap and lead for splits, corrosion or a loose connection.
A simple spark test can help, but work safely. Keep fuel vapour away, use an approved ignition tester where available and do not hold the plug against the engine with bare hands while cranking. No spark can indicate a coil, kill-switch wiring, flywheel key or ignition-module fault rather than a failed plug.
Treat a spark plug as both a replaceable part and a diagnostic clue. Fit the exact specification, keep the service interval realistic for the work the machine does, and investigate any plug that fouls again quickly. That small check can keep the mower, generator or pump ready for the next job rather than waiting on the bench.