When you take out your spark plugs, if the colour of the electrode is red to yellow, brown or grey your engine is correctly tuned and the plugs are in good working order. There should also be hardly any combustion residue.

(Taken from the Skipper’s Outboard Motor Guide, by Hans Donat, and published by Adlard Coles Nautical, ISBN 978-1-4081-1418-6 – a fantastic new waterproof engine handbook, available from all good nautical bookshops)