In order to run, any internal combustion 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.
There is no fuel left in the tank
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.
One or more cylinders has low compression
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.
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.
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 fuel in the tank is old and has gone stale
Petrol doesn't last forever. It is generally considered to be too old by the time it is about 6-12 months old, unless treated with an additive.