Mercury 402 40hp - Thermostat & Telltale pee hole

[Welshy613]

Hi, Wondering if anyone can answer a question for me?
I picked up a used boat and Mercury 402 40 hp motor last week. Serial #7070514

I was aware it had problems and plan on trying to get it running smoothly again.
The first problem I'm trying to solve is the lack of water coming out of the telltale.
While the lower end is submerged in water it only drips water from the telltale with the motor running.

I've installed a new impeller and can confirm that it's working.
Check the telltale end for blockages but it was clear.
I checked the thermostat and found it had failed and was stuck open and damaged.

If the thermostat is stuck open would that prevent water from coming out of the telltale end?

Before screwed thermostat cover/part back down I started the engine for a few seconds to see if water was being pumped.
Water came gushing out.

If the thermostat doesn't make any difference with water coming out of the telltale, what would be your next step at finding the clog/problem?

In the uploaded image you can see what I believe is the poppet valve, should it be very hard to push up on the spring?

Any advice or opinions are much appreciated.

 

[Texasmark]

15 or so psi water pressure at WOT so it doesn't take much to upseat the poppet unless stuck. Follow the TT hose back to it's source. Could come from the exhaust maniflod, port side of engine flat plate on block. If so, different cooling circuit...stat and poppet not controlling that. Unscrew the hose adapter and run your flow test again with it out.

 

[Welshy613]

Thanks for responding.
Would this be the exhaust manifold?

 

[Texasmark]

Right place, looks like it....never personally looked at your engine.... can't see all of it and see no hose barb and hose to the TT. Go to the inside of the TT connection on the cowling and follow the hose.