Scuba need your expertise. I am looking at a boat with a 120 merc in order outbourd it is running fine at idle but want to over heat under load.

Have you checked the impeller pump ?

It should be pumping a steady stream of water out the back.

You can buy a gauge to put in the dash to check the water pump pressure. Easy to install, a hose runs back to engine.

So if anyone has any advice on this please feel free to give it. I am thinking that there is a bloage in the outlet for the water because it runs fine at idle?

Pump is working fine the guy said it is new and is at factory flow I am beginning to think that there is a blockage in the output of the moter where should I look?

You’re probably going to have to check multiple things before you find the problem. Poppet, thermostat, exhaust manifold seal/gasket.

Scuba may not see this post. Click on the chat forum above and you'll see he has a thread going called Camp Scuba. He visits that thread daily. You could ask him there.