In order to run, any internal combustion engine requires three main components clean fuel, compression, and a well-timed spark. It is helpful to keep these three things in mind when going through the diagnostic process.
In this video I go through how I approach an outboard that turns over but won't start, checking for fuel, compression and spark to narrow down which of the problems below is the cause.
It's always important to start with the basics. Sometimes boats use fuel faster than you expect and you may just be out of fuel. It is also possible to have fuel leaks or even have fuel stolen so be sure to check this first.
In order to run and start the pistons must be able to compress the air drawn into the cylinder sufficiently to get the correct air to fuel ratio needed to start and run.
A very common reason that outboard motors run badly is from dirt 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.
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 you may not be getting a good spark.
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