Fuel tanks need a way to let air in and out of them as the fuel level changes. This is the job of the fuel tank breather.

A faulty pump can starve the engine, or a torn diaphragm on a two stroke can push fuel into the crankcase and make it run rich. I rebuild one with new check valves, diaphragm and gaskets.

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.

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.

If a two-stroke outboard is run on pure unleaded petrol instead of petrol mixed with oil, the engine will overheat and seize causing it to stall.

Engine stalls at idle or clunks when shifted into gear. I set the idle mixture screw by ear, then the idle speed screw, once the carburettor is clean and the float level is correct.

When fuel pressure drops too low, an EFI outboard won't run properly. I test pressure with a gauge, then strip the vapour separator tank to check its strainers, high pressure pump and regulator.