Johnson 25hp Overheating?

I'm working on a 2002 Johnson 25 hp 2-stroke. It came with a boat a just picked up that had been sitting a while so I don't know much about its history. I'm going through it trying to get it back up and running - but I'm having a problem I just can't figure out. It then runs fin for about 10-15 minutes, but then the water stops coming out of the tell tale and it starts to overheat. Also, it takes a bout 20-30 seconds after start up for water to start coming our of the tell tale when I first start it cold.

The first this I did when I got the motor was replace the the water pump (housing, impeller, etc). What would cause water to stop coming out of the tell tale once the motor gets hot? I'm thinking it may be the thermostat so I have one on order just in case, but I don't want to throw parts at this. If I let the engine cool off, the water will flow correctly again, but once it heats up it stops. I know my way around engines pretty good, but I'm new to outboards. Any help would be greatly appreciated. Thanks in advance!

Thanks, I will check that. It’s was a an OEM replacement water pump and the tube for very snuggly - I was pretty tough to get everything in and lined up. It’s possible that something may have slipped out of place.

Remove the lower unit and the thermostat, stick a garden hose in the thermostat housing and flush the motor from the top down. Ants and mud daubers love those little orifices. If there is any junk in it, chances are the thermostat will have caught quite a bit of it.

Also pull the grates on the lower unit and make sure the water passage from the pickup to the water pump is clear.

Ok, I checked the tube and grommets and they all looked good. I had planned on replacing the thermostat anyway so I did that and flushed everything out. I put it all back together and I’m still have the same problem - the water pumps fine when the engine is cold but as soon as it get up to temperature the water stops. It doesn’t matter what RPM, it’s either all or nothing. Any other ideas?

Have you tried taking the thermostat cover off when it stops pumping, & start the engine - careful, could get a little wet! Maybe just loosen the screws.

That would tell you if the blockage is before or after the thermostat.

Have you checked the impeller key, maybe it is partially sheared.