The fact that they are “immerse” in the combustion chamber means that the conditions of the spark plug can tell you a lot about the overall conditions of the engine. text M. Voltini © Archivio Vroom
In particular they can tell you if carburetion is good or not. You can see this from the colour of the insulation: if it is light hazel nut colour it is good, if it is dark, probably carburetion is too rich (the contrary if it is nearly white). If you see any trace of oil, then it is obvious, you have ignition problems; it is a rich mixture especially at low revs. Also if the engine doesn’t start, the spark plug can give you an idea the reason: if it remains untouched, as if it were new, then it means that the petrol doesn’t get there; if, instead it is wet, there is too much fuel or you have ignition problems. Another thing that you can do when the spark plug is mounted, is check the inside of the cylinder through the bore, use a flexible torch to help you: this too, the colour of the piston crown can help you to understand if you have the right carburetion (should be tobacco colour) and then you can check to see other problems, should there be any.