Outboard > Hard to start
This topic covers a variety of reasons that an outboard motor can be hard to start. If your engine is always hard to start when hot or cold, the first two options will give you information specific to those two situations.
Outboard motor is hard to start from cold
It is most common for outboard motors to be hard to start when they are cold and have been sitting for a long time.
The outboard is hard to start again after running
The outboard has faulty reed valves
Only two-stroke motors have reed valves. These are one-way valves that stop the air and fuel mixture coming back out from the crankcase.
The fuel tank breather vent is closed
Fuel tanks need a way to let air in an out of them as the fuel level changes. This is the job of the fuel tank breather.
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.
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.