When you take out your spark plugs, if the colour of the electrode is dark brown, your engine is driving too slowly, the carburettor is delivering a mixture that contains too much oil, there is too much oil in the fuel, the mixing system is not working correctly, the ignition tension is too low, or you have the wrong spark plugs (the heat value is too high).

(Taken from the Skipper’s Outboard Motor Guide, a brand-new fantastic waterproof handbook by Hans Donat, published by Adlard Coles Nautical, ISBN 978-1-4081-1418-6 – £10.99, and available from all good nautical bookshops)