It’s important your spark plugs have the correct distance between the electrodes – you can use a feeler gauge to determine the gap distance. You can then adjust it by carefully bending the electrode. You can find the correct gap distance in the manual.

(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)