Have you ever measured the output of your waterpump ? If your engine isn't clogged, you can measure it at the outlet from the port side of the cylinder block (Part #22 in Fig. 11 of the Spare Parts Lists). The Webmaster's engine, with a two year old impellor, pumps about 2.7 litres per minute when idling at 1000 rpm. Cruising at 2400 rpm, that works out at about 6.5 litres per minute. If you have measured the output of your pump, please E-mail the Webmaster with the information.
This is a very easy job - but only if your installation resembles the one on the left. Most of us, unfortunately, have something like the one on the right. Was there a sadistic streak in the designer who decided to locate the water pump at the rear of the engine, below the propellor shaft?
I've heard from one guy who enlisted the aid of his friendly dentist to replace the impellor and even the water pump seals, using mirrors. Some others have been able to improve access sufficiently by removing the exhaust gas collector. But most find it easier in the long run to remove the engine completely.
In the photo at left the engine has been removed from the boat and stood on it's end, so that it is resting on the flywheel. The back of the exhaust gas collector is just visible to the right of the photo. The clear hose, which is slightly kinked, joins the pump to the bottom of the exhaust gas collector. In the photo, one of the bolts has already been removed or is missing. Replacing the impellor is a chore that you'll do more than once in the engine's life, so take note of Iain's tip: -Water pump access is easier if the securing screws are replaced with 5M hexagonal-headed A2 stainless studs. The original cross headed screws get into a sorry state rather quickly.
When removing the pump, take care not to lose the small Woodruff key (Spare Parts List, Fig. 18, #24) from the pump driveshaft. In the right hand photo the pump housing has been removed, as has the impellor and the backing plate (Spare Parts List, Fig. 16, #5); the backing plate is still lying on the back of the gearbox beside the shaft that drives the pump. The backing plate often stays stuck to the back of the gearbox after the water pump housing has been removed, which can be a bit confusing if you are trying to replace the water pump seals - you can't see them. If you're only replacing the impellor, don't worry about it - there is no need to remove it.
The parts are shown in the photo at left. You don't have to use "genuine" Vire parts; I've heard from the USA that the impellor for some Mercury outboards can be used and Fairways Marine, the Vire agent in the UK, will also supply a cheaper impellor on request. Visit the Alternative Parts page for details. Veli-Antii, who supplied most of the photos, also says that you can make your own gasket (middle photo). Once you've managed to get decent access to the water pump, disassembly and reassembly takes about five minutes, unless you intend replacing the water pump seals. In this last photo the engine is upright again, the missing screw has been replaced, and the bit of transparent hose has been replaced with a short black hose.
Maybe you're a complete masochist, and you really, really don't want to remove the engine. If so, then here's Bill Hoover's notes on how to do it the hard way: -
This pump will need dismantling from time to time due to the impellor disintegrating (I've never had it happen in over 25 years) or the brass bushing becoming unbonded from the rubber impellor (I've had this happen twice). In either case, the pump is a bitch to work on and even the factory techs prefer to remove the engine to replace the impellor. Here is how you do it, engine in the boat.
PROCEDURE - DISASSEMBLY
The above is the easy part.
PROCEDURE - ASSEMBLY
Assuming that you have (at least) two new gaskets and a new impellor...
I suggest a trial assembly, with no gaskets, to get the feel of it and to get the impellor lined up with the keyed drive shaft. It's the gaskets that make assembly so hard.
If you are satisfied that you are master of this operation, then you are ready for final assembly, with gaskets.
Remember, the gaskets are not symmetrical; they have a long side and only fit one way. Assemble the whole pump with both gaskets to assure you have the gaskets oriented and all holes lined up.
You are done. Reattach the intake hose and open the seawater valve. Start the engine and check for pump working and water leaks at the pump. If you have a gasket leak, tightening pump bolts won't fix it. A small tweak on the outlet hose might seal that up, if required.