1983 Yamaha 30EN Outboards Service Manual, Page 84Get this manual
LOWER UNIT OVERHAUL OEseR IPTION The exhaust gases forced out of the engine flow through the exhaust pipe in the engine and expand in the upper casing, in which the exhaust gas pressure is decreasedAt low speeds, the exhaust gases flow through the labyrinth provided in the upper part of the upper casing and stream out into the airAt normal cruising speed, the cooling water in the upper casing is forced out by the exhaust gas pressure and rotary motion of the propeller and as result, the exhaust gases stream out of the propeller bossThe cooling water inlet is provided in the gear case, and water is sucked by the water pump located in the upper part of the gear case and supplied to the engineThe cooling water circuit is an open type and thus, the engine is always cooled by new waterThe impeller in the water pump is keyed to the drive shaft so that the water flow rate is controlled depending on the engine speedThe gear case has both forward and reverse gears (spiral bevel gears) and clutch dogBy sliding the clutch dog back and forth by moving the plunger, the rotating direction of the propeller is changedIf disassembly of the lower unit or bracket unit is required, it is necessary to remove the powerhead assembly(Refer to powerhead removal page 4-1