Spark Plugs...

In this post we'll have a look at the humble spark plug. We often don't even give it a second thought until it starts to go wrong and then change it. But it can give us a clue as to the state of the engine tuning, and we can maintain peak engine performance with just a few minutes simple maintenance. Find out more after the jump...

There are a number of simple checks to make once you have removed your spark plug - there's not a lot of actual maintenance to do to a spark plug, but consider the following list:

  • Check for cracks or damage in the ceramic insulator.

  • If your plug has a screw-on cap, make sure that it's done up tight (not too tight though!).

  • Ensure that you have the correct type of spark plug if you are replacing it. You should be able to verify the type by referring to your owners manual (you did keep that in a safe place didn't you?).

  • The colour of the electrode will give you a clue as to the state of engine tune; it should be a sort of grey-brown. We'll take a closer look at electrode colour in a moment.

  • The gap between the electrodes should be set and checked using your thumbnail feeler gauges, the 'normal' setting tends to be around 0.5mm - but you must check this for your own saw. The feeler gauges should be a reasonably tight fit between the electrodes.

  • Electrode wear occurs due to the constant sparking eroding the electrodes; initially the electrodes become pitted and then start to loose their square ended shape as the erosion worsens.

Plug Colour

As we've just mentioned, the colour of the plug's electrodes give a clue as to what state of tune the engine is in. Under normal conditions the electrodes are a grey-brown colour (some people refer to this as 'biscuity-brown', but that's total nonsense as a Bourbon is quite a different colour from a Custard Cream).
If your plug has dark brown / black deposits on it, then it's a sign that the mixture of air:fuel is out of kilter. In this instance, there is a lack of air in this air:fuel mixture, and this can be caused by a clogged air filter. In fact, you should never make any adjustments to the carburettor with a dirty air filter.
At the other end of the extreme, you may be facing light brown / white deposits on your plug. If so, it's a sign that the engine is running weak and there is too much air in the air:fuel ratio - it's also a dangerous state for the saw to be in and you should get it checked out quickly. This can be a problem with saws that are constantly being tinkered with, as this situation is commonly related to a mis-tuned carburettor.


When refitting the plug, many people do them up too tightly - wrenching them up very tightly; they should be finger-tight... then just nipped up with the plug spanner. All you need to do is to slightly squeeze the compression washer fitted to the spark plug to create a seal that won't allow the gases to escape when the saw is running.
That's all for our look at spark plugs, there will be another maintenance post along shortly...

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