Spark plug maintenance: electrode gap, colour and wear

When to fix them yourself and when to contact a service centre

Inspirations / How to


Estimated reading time 2 minutes

The spark plug is the heart of the ignition system on petrol-engine garden tools, including brushcutters, lawnmowers, chainsaws and blowers (to name just some of the most essential tools in any garden shed ). This is because it generates the spark that ignites the fuel-air mixture, thus triggering the combustion process that makes the engine work.

The spark is produced between the central and ground electrodes. The distance between the spark plug electrodes needs to be checked, as a spark may not be produced (at all, or occasionally) if the electrodes are too far apart. On the other hand, the spark may be too small if they are too close together. For every spark plug there is an optimal electrode gap, which is specified by the manufacturer. You can find this parameter in the user and maintenance manual for your garden tool: for Efco machines, the standard spark plug electrode gap is 0.5 mm for chainsaws, brushcutters and blowers, and 0.7–0.8 mm for tools such as lawnmowers.

So, when performing maintenance on a petrol engine gardening tool, after cleaning the spark plug with a cloth, check the distance between the electrodes with a blade/roller feeler gauge and adjust it accordingly by gently moving the ground electrode nearer to or further from the central electrode.

When the electrodes wear out, their distance increases to the point of no longer being adjustable, at which point the spark plug should be replaced because it has reached the end of its life cycle. Check the user manual for the recommended replacement part.

Look out for engine health signs as indicated by the appearance of the electrodes and the colour of the spark plug (specifically, the colour of the insulator around the central electrode). Also observe whether the carburettor is working normally, whether the engine tends to overheat, whether the fuel/oil mixture in 2-stroke engines is prepared correctly, whether there are problems with the oil in 4-stroke engines (look out for oily deposits on the spark plug) and so on.

Another cause of spark plug fouling is flooding of the engine (an unmistakable sign of this is the smell of petrol). It happens if too much fuel has entered the combustion chamber due to pressing the primer too many times before starting: you can rectify the situation by following the anti-flooding procedure provided in the manual.

Getting back to the spark plug colour, if the spark plug electrode is white, it may mean that the engine is running lean (not enough fuel) or that the engine is overheating; if it's black it indicates that the engine is running rich (too much fuel). In these cases, the spark plug should be replaced, but first you need to check and resolve the problem, depending on the colour of the spark plug: for Efco gardening tools, contact your nearest authorised service centre.

On the other hand, if the spark plug electrode is brownish/grey it means that the fuel/air mix coming out of the carburettor is balanced and that the engine is fine overall.

Earlier we mentioned routine maintenance of petrol-powered tools which, among other tasks, involves checking, cleaning and replacing the spark plug (this should also be done before winter storage): here you will find detailed instructions on maintaining brushcutters and maintaining blowers.

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