Generically speaking, there are three methods to flush an
outboard: using the freshwater flushing attachment built into the outboard, flush bags [see pic], or flush muffs.
More ➥ https://yamahaoutboards.com/blog/mai...of-an-outboard
According to BoatUS: Yamaha gives three choices — with a flush bag, muffs, or hose-port connector and says
all three methods work equally well.
Bag and Muffs: Engine should be vertical, run no more than 800-900
rpm in neutral for 15 minutes with the prop removed. Be sure the engine is receiving
cooling water (by checking the
overboard indicator). If your outboard has more than one set of cooling inlets, a flush bag should be used.
Flushing Port: Engine should not be running; it can be tilted or vertical. Prop should be removed. Flush for 10-15 minutes.