The function of the spark plug is closely linked to that of the Vespa. We can imagine the driver of a Vespa, sweating and tirelessly pushing the kick starter, but his scooter just won’t start, as a symbol for a faulty or defective spark plug. To ensure that the Vespa always starts without complications, it can be quite helpful to clean the Vespa spark plugs.
Electrode gap as the decisive distance
To ignite the mixture of fuel and air provided by the carburettor, a centre electrode and a ground electrode are required. The distance between the two electrodes is called the electrode gap. This varies depending on the type of spark plug and in most cases is between 0.3 and 0.6mm on Vespas. This distance should be set to the appropriate distance for the model using a suitable feeler gauge.
The ignition voltage finally produces a spark between the two electrodes, which ignites the fuel-air mixture in the combustion chamber. Very important when buying a spark plug is the appropriate heat value of the electrode. A spark plug must reach the so-called free-burning limit as quickly as possible after starting the scooter. At a temperature between 400-450 °C, residues from the mixture are burnt free at the electrodes.
Engine running too rich or too thin
A spark plug is also the ideal indicator of how the engine is running. Ideally, the spark plug should be fawn-coloured and have no deposits. If there are residues of soot or oil on the insulator of the plug, it can be assumed that the mixture contains too much oil. This is colloquially referred to as “rich” engine running. In contrast, the engine runs with too little oil if the spark plug is relatively light.
This test can easily be done by unscrewing the spark plug immediately after shutting down a warm engine.
Removing and installing the spark plug
To be able to check the spark plug at all, it is of course always necessary to remove it. On Vespas, the spark plug is in the cylinder in almost all cases under the right-hand side cover. First, the spark plug cap must be carefully removed. Now you can already see part of the spark plug. A suitable spark plug key is the ideal tool for removal. With this, the spark plug can be easily unscrewed with the hexagon nut on the plug housing. Possible residues around the spark plug should be removed in advance, as residues falling into the combustion chamber can quickly lead to piston sticking.
The installation is done in reverse order. For installation, professionals rely on a torque spanner with a suitable attachment. This allows the spark plug to be tightened to the appropriate torque. If a torque spanner is not available, the plug can also be hand-tightened before adding another 3⁄4 turn with a spanner to ensure optimum hold.
Cleaning the Vespa spark plugs
If a sooty or bridged spark plug is removed during a regular check, it can be cleaned relatively easily.
A simple spark plug brush is recommended for this. The deposits should be removed from the electrodes with slow and moderate strokes. Afterwards, the thread, which is usually also dirty, can be cleaned carefully. Special care should be taken when applying pressure to the earth electrode. This is very sensitive and can therefore be bent easily. This reduces the all-important electrode gap and the ignition spark can no longer jump over.
A more elaborate form of cleaning is ultrasonic cleaning. The vibrations of the ultrasound remove even the most stubborn residues without any problems.
When is it worth changing the spark plugs?
Of course, cleaning with the two above methods is not always sensible. If, for example, the centre or earth electrode already has rounded edges, it is advisable to replace it. In general, manufacturers always specify intervals for replacement. However, it always depends on how you drive. If the Vespa was always driven on short distances, on which the free-burning temperature was possibly not always reached, then a change is recommended earlier. Since spark plugs are relatively inexpensive anyway, especially for the smallframe models, the premise applies: It is better to change them earlier than to be annoyed later.
Finally, a small expert tip: A spare spark plug should always be in the tool compartment under the seat. In an emergency, it can be changed quickly and will quickly revive your enjoyment of your Vespa.