Thanks Mike,
I pulled both risers and boots.. The flaps are intact and in very good condition.. Like new.. Both fold without any issues or obstructions.. Put it back with new gaskets like you suggested.
When firing the engine back up with the muffs, looking at the back of the boat, water eventually comes on the port side from the transome however its just smoke/exhaust and no water at all coming from the starboard side. No water coming out the prop either.
I took the thermostat housing off as I was curious how it worked and replaced the thermostat with a new one while I was there.. Everything looked clear with no obstruction. Stuck the old thermostat in boiling water and opened just fine. Also checked the pipes from the thermostat to the bottom of the manifolds.. All clear there too with no gunk. I did a quick drain using the drain plugs at the bottom of the manifolds, water did come out both sides. Very hot from the starboard side and cold/lukewarm from the port.
I did some IR thermal readings when the engine got up to temperature @ 80c:
Port side top of rise 25c
Port side top of manifold 27c
Port side side of manifold 26c
Port side side of rise 26c
Port side top of engine beside rise 20c
Starboard side top of rise 84c
Starboard side top of manifold 90c
Starboard side side of manifold 91c
Starboard side side of rise 91c
Starboard side top of engine beside rise 36c
Thermostat housing 48c at joint on engine, 59c at bowside. Bottom of engine at block 59c/64c.
My Dr Watson gut feeling: I wonder if its the impeller thats gone? perhaps its not getting enough flow to push to the other side of the engine / manifolds? The muffs are ok and pressure is good from the tap. Trying to find a bucket big enough to fit the outdrive is difficult.
Next steps? Pull the outdrive and replace the impeller? or anything else I can do before trying that?
Haven't done an oil change or spark plugs yet.. Will get round to that after cooling is sorted :-)
Thanks again