I have a 2005 40 Yamaha 2 stroke that's not pumping water. Changed the water pump and have taken thermostat out and it's still barely pumping.

SOURCE: Overheating engine

Check your oil dipstick for coolant, or a mucky substance. Your symptoms sound like a blown head gasket.

SOURCE: 99 grand am gt overheating changed thermostat and

try priming the engine block & radiator with water though the hose's & any hose you can squeeze to pump out the air lock with the filler cap off when running, sounds too much air is is in the system & the pump won't work dry.
I had this problem a few times & I ended up practically forcing water into any hose I could re-attach quickly using a garden hose, as the heating system need's priming too !

SOURCE: Overheating....changed waterpump/thermostat!

if your heater is working i would say you got a bad thermostat if it isnt blowing any hot air you probably have a blown head gasket

SOURCE: Water pump is leaking on my 2001 Yamaha YZF 426 motorcycle.

First you remove the four 8mm bolt that attach the water pump cover to the side cover, the lowest bolt will drain the coolant. next you remove the coolant pipe with one 8mm bolt. The water pump cover like any part that has been in place for years may need some force to remove and that is why you will find two tabs around the outside of the pump cover for prying it off.

Once you have the cover off the impeller unscrews by way of a built in 10mm nut. This nut is right hand threaded and is removable by rotating it counter clockwise with preferably a 6 point socket as a 12 point socket may round the nut sides since the impeller and built in nut are made of aluminum.

To replace the twin seals (one for water/coolant and one for oil) you must remove the engine side cover from the motor and pull the impeller shaft out from the engine side of the cover. After the shaft has been removed the bearing is pulled out with a bearing puller or sometimes it can be removed by pushing it out from the outside in with a makeshift driver but the bearing can be damaged by this method and it is not recommended. The old seals can now be pulled out or pushed out from the opposite side.

To replace the new seals you must drive them in perpendicular to the impeller shaft with the manufacture markings to the inside. The water seal is installed from the outside and the oils seal from the inside.

There is a tool used for installing the seals called a seal driver but you could make one in a pinch. The seal driver makes contact with the outside diameter of the seal where it is solid and has a metal core. The inside diameter is soft and flexible and should not be used to push the seal in place however it can be used to keep the seal driver centered over the outer diameter and to keep the seal moving in a strait line perpendicular to the impeller shaft.

Once the new seals are in place they should be greased with some lithium soap based grease.

Install the bearing and reassemble the impeller shaft then you are ready to reassemble the engine side cover and water pump housing but make sure you spin the water pump impeller slowly as you install the engine side cover as the impeller shaft is driven off the engine shaft behind it and the shaft needs to be aligned in order for it to fall in place (do not try to force the cover on).

SOURCE: i have 30 hp two stroke pull start yamaha outboard

yam water pumps are clever the offste centre causes the pump vanes to flex during rotation and at low RPM you have a displacement type pump, at high RPM water pressure forces the vanes to flex inwards and the pump becomes centrifugal, you can fit the impellar either way up so you cant fit it wrongly make sure your gaskets are ok and that the delivery pipe to the power haed is not leaking (try a hose on it) also make sure the little woodruff key or on some models the pin through the drive shaft which turns the pump is fitted ok, not much else that can be wrong!