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.