Water not Pumping out of tell tail , checked all the usual suspects , help Please !

I have a really clean Nissan 30HP 4 stroke NSF30B . Has been cared for just not used much lately due to being overworked at my job . I do hook the ears up and start the boat every 2 weeks . water seemed weak lately so figured good time to install new impeller, All has been working good the last few weeks but now all of a sudden no water at tell tail. checked output tubes and nothing blocking. I have pulled the thermostat and looks bad so i will replace it, but I did put it back together without thermostat just to check but still no water . I did fire the engine up with the thermostat cover off and plenty of water flow! Could I have a clog inside the engine ? If so are there any tricks to unclog without pulling the head ? Thanks for any advice !

Stat will not affect pressure.
Never do just an impeller; always do a complete kit.
Check the nipple where the pee tube joins the motor. Pust a stiff monofilament in it while the motor is running. Probably just some crud in there.

Thanks for the information about the stat ! I will double check the inlet nipple this weekend when I can get my hands on the motor again . I hope the problem is that simple It would make me very happy to see water pumping out of the little engine again , thank you !