Motorcycle spark plugs: troubleshooting –

The motorcycle’s spark plugs are responsible for making the spark that will make the engine run. Sometimes they cause problems and prevent the engine from starting, dirt, faults in the connection with the electrical system or that they have reached the end of their useful life can be the causes.

Motorcycle spark plugs

The motorcycle’s spark plugs are a good reference to detect problems . Its disassembly and color will tell us by themselves the state of our engine, the type of combustion that is developing inside. But first we go with some basics of what spark plugs do.

Both a two-stroke and four-stroke engine need an element to trigger combustion . When the piston is about to reach the highest point of combustion, with the cylinder already full of a mixture of air and gasoline, the spark plug is responsible for launching a powerful spark that causes the mixture to burn. This moment is very important, it must be at the right time for maximum effectiveness.

Actually the engines work with an ignition advance so that the spark is launched a few moments before the maximum point of combustion so that at this moment the explosion has already been triggered. Now with the arrival of electronic injection it is easier because the ignition system takes into account the revolutions at which the engine is running and adjusts the exact point of ignition. In old engines it was done in a fixed way.

The spark plugs are screwed into the cylinder head from where they take the (electrical) mass of the motorcycle. At the other extreme we have what we know as  the pipe , from where they take the active pole of the electrical system with which the spark will be formed. Thus, when detecting any breakdowns or problems with the motorcycle, we will have to check the condition of the pipe . That it makes good contact, that it does not receive humidity from the outside after a rainy day or that it is not deteriorated and that it communicates (make contact with a part of the motorcycle that derives electricity) the two poles of the electrical system.

Check it well, that it is clean and, above all, that it does not show samples damaged by the heat of the engine. Wow, it didn’t get burned. The pipe also has rubber caps on the cable and in the housing with the upper part of the spark plug, whose function is that water or moisture does not leak inside.

To gin, a spark plug wrench

If our engine does not start, or one of the cylinders does not work, it may be because one of the spark plugs is giving problems. To see if they spark with the spark plug connected to its pipe and on the cylinder head, it should give a spark when we move the engine. If not, we have a good clue to investigate. Depending on the motorcycle, some spark plugs are more complex to remove than others (remember that they are on the cylinder head). To disassemble it we will need a spark plug wrench, and these can be of three different sizes … On a motorcycle, the most normal thing is that they are 16 mm. You also have 18 mm. and 21 mm., which are more common in cars. The most normal thing is that the spark plug wrench is articulated that allows us to adapt to the tortuous spaces of some motorcycles. They will have to be longer or shorter depending on the depth of the well where they are located.


There are motorcycles in which the accessibility of the spark plugs is a pleasure, but others that will make things more difficult for us, that is why we will find that some users have never changed the spark plugs and have not respected the replacement periods, giving performance problems of these engines. When this happens, it is best to buy the specific spark plug wrench for our motorcycle, which we can buy at any brand dealer.

Look at the color of the spark plugs on your motorcycle

The color of the spark plugs speak of the state of health of our engine. Broadly speaking, a dark colored spark plug means that the mixture is very rich, that it has a high proportion of gasoline and, as you well know, excess gasoline means poor combustion. It may be because the motorcycle does not normally circulate on long distances, the engine does not warm up properly and the self-cleaning effect of the spark plug does not occur.

In the opposite case, we find a light-colored spark plug , which means lean mixture, too much air, and that the engine runs at very high temperatures. We come across other factors, such as oil-dirty spark plugs (oil enters the combustion chamber), broken porcelain, deformed or broken contact ends.

Depending on what we find, it will give us a clue of what to check, in parallel with our engine. But the basic and normal is to see that the spark plug has a somewhat blackish appearance as a result of the particles and ash that has accumulated during combustion.

For cleaning we will use a metal bristle brush , with which we will remove the carbon. We will check the separation between the electrodes according to what the manufacturer recommends, but as a general rule it should be 0.7 or 0.8 mm. We can measure it with a thickness gauge (a set of metal plates with different measurements that we can find in specialized establishments for very little money).

In the case of doubt or that the recommended period has arrived, we will look for new spark plugs respecting their thermal degree , we will see it in another article, but it refers to the amount of heat that it transmits to the cylinder head. Do not change it and always use what the characteristics of your motorcycle recommend.

A change of spark plugs and air filter is one of the simplest and most noticeable checks on a motorcycle, we recommend that you do them.



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