How do you flush a Yamaha outboard motor?

To flush a Yamaha outboard correctly, use the built-in flush port with the engine switched off, or place flushing muffs over the lower water intakes with the engine idling. Both methods require running fresh water through the cooling system for 10 to 15 minutes to dissolve salt deposits and prevent internal corrosion.

Built-In Flush Port Method

Modern Yamaha outboards from the F9.9 up to the V8 F350 feature a factory-fitted flush hose connector on the side of the cowling or midsection. This method suits boats kept on a mooring or a pontoon where reaching the lower unit proves difficult.

Never run the engine when using this built-in port. The water enters the cooling galleries above the water pump. The rubber impeller in the lower unit will run dry and burn out within seconds.

• Tilt the engine fully down so water drains naturally. If the boat remains moored and tilted up, the flush still works, but tilt the engine down later to drain the system.
• Unscrew the flush fitting, connect a garden hose, and turn on the fresh water tap to normal mains pressure.
• Let the water flow through the engine for 10 to 15 minutes. Water will cascade out of the lower unit and the propeller hub.
• Turn off the tap, disconnect the hose, and tightly secure the cap. A loose cap draws air into the cooling system during operation and causes overheating.

Flushing Muffs Method

Flushing with muffs provides a thorough clean for engines kept on a trailer or dry stack. The running engine opens the thermostat. Fresh water circulates through the entire engine block and washes the water pump impeller.

Remove the propeller before running the engine on muffs on dry land to prevent severe injury.

Buy a set of dual-feed flushing muffs that clamp tightly over the water intake grates on the lower unit. Single-feed muffs often leak water and starve the water pump.

1. Tilt the engine fully down.
2. Slide the muffs over the primary water intakes. For larger V6 or V8 Yamahas (like the F250 or F300), locate the secondary water intakes on the nose of the gearcase and cover them with duct tape to maintain water pressure.
3. Turn the hose tap on full blast. Water must squelch out heavily from around the rubber cups.
4. Start the engine and leave it idling in neutral. Never rev the engine on muffs.
5. Check the tell-tale stream immediately. It must flow strongly. If the stream appears weak or absent after 10 seconds, shut down the engine and reposition the muffs.
6. Run the engine for 10 to 15 minutes, then shut it down before turning off the tap.