Prime Start replaces the choke on some carburetted Yamaha outboards. It is a valve with a wax element that opens an extra fuel passage when the engine is cold. Once the engine is running, the charging system powers a small heater in the unit. This warms the wax, which expands and gradually closes the valve.

If the valve sticks closed, the engine is hard to start from cold. If it sticks open or the heater fails, the engine will run rich once warm. The fuel passages can also block with old fuel, the same way a dirty carburettor does. The following video goes through removing, diagnosing and servicing the Prime Start system.

A tip from a viewer with a Yamaha Pro 50 that would only start from cold with the emergency start valve open, even after its carburettors had been cleaned: the Prime Start valve wasn't pulling its pin far enough out of the jet. The valve is screwed into the carburettor, and Yamaha glues the threads once it's set. Carefully breaking the glue let him adjust how deep the pin sits, and that fixed it. Shimming the valve might pull the pin back without disturbing the threads. It's worth trying only once the valve itself and the fuel passages are known to be good.