2005 5hp 4 stroke cooling problem

[charlus]

Here is a quiz regarding a cooling problem on a 2005 4/5hp 4stroke:
At idle water circulates but with less than normal flow and very hot,
When rpm increase the flow diminishes until no more water comes out from the telltale spout (eventually steam comes out)
If the side rubber cover (giving access to the shift rod connection)on the middle unit is removed the flow instantly returns to normal (in flow and temperature) and everything runs fine (but noisy!) .
Obviously the water flow is interrupted as soon as there is a counter pressure due to the exhaust inside the middle unit.
-Underwater exhaust outlet not obstructed
-The Power head/middle unit gaskets have been changed
-The water tube is perfectly set in the bushing at the pump and in the upper part
-Everything in the water pump is new. Shaft O ring checked.
-There is no trace of exhaust gasses intrusion in the water jackets of the power head.
Nobel prize given (plus all my thanks) to the person who finds the solution.

[JUST-IN-TIME]

new waterpump housing also?

[charlus]

Yes, the water pump is new. I think I tried everything without success. Obviously as soon as the exhaust gasses reach a certain pressure in the middle housing they stop the flow of cooling water. How? that's the question.

[JUST-IN-TIME]

you got a clogged exhaust turner then

drop lower unit and look