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Outboard Guide
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Sometimes an engine will start or run, but cough and splutter instead of running smoothly and strongly.

There is bad or no spark in one or more cylinders

Without a strong, well-timed spark an engine won't start and run. This section covers how to test for spark and the main reasons why your may not be getting a good spark.

The carburettor has debris blocking the jets

A very common reason that outboard motors run badly is from dirty in the carburettor(s). This can be from water or other contaminants in the fuel, perished fuel lines breaking up and putting debris into the fuel or varnish from fuel that has evaporated after the engine has been sitting for a long time.

The head gasket on the engine is blown

There is a gasket that seals the cylinder head to the engine block to prevent the combustion gasses escaping and to keep the cooling water (and oil on a four stroke) out of the cylinders.

Synchronising the carburettors on an outboard will make it run smoothly

Outboards that have multiple carburettors need to the carburettors synchronised in order to run smoothly.