Can you see any water discharging from the blubber hole on the back of the motor?   Some things to look at:   Did you seal the impeller plate to the gearcase housing, if not the pump may not pump water correctly.   When you replaced the impeller, was the old impeller intact or were pieces broken off/missing?   Maybe a piece of the old impeller or some other obstruction in the water tube from the pump up to the powerhead.   Maybe an obstruction somewhere in the water jackets in the powerhead.    When you replaced the impeller did you also replace the impeller plate and pump housing?   Sometimes a new impeller with an old impeller plate or old pump housing won’t pump correctly.   Did you look to see if the air bleed hole in the pump housing was clear and not blocked?

Yeah, the big twins don’t like to cool right in a barrel, for some reason. They pump a lot of their own exhaust into the cooling system, it seems.

John O’Shea wrote:

Thanks to all for the quick response.

Frank, I will take a look at your suggestions.

I have rigged, a homemade adapter plate that connects right to the intake, hooked  right to a garden hose, which I run at a low velocity

 

 

Does that mean a plate that replaces the bypass plate on the side of the gearcase?  “Low velocity” might be a clue.  You are leaving a water/air path to/from the water intake screen aft of the prop.