"Guy,good idea I will try this
"Guy,good idea I will try this if the hoses are long enough next time.
Update after spending part of July 4th at the dock repairing the boat. I am encouraged but not totally out of the woods yet. First,I took the thermostat out as MikeM did and both risers stayed cool and engine gauge stays lower at 130 degrees. My engine is responding the same as Mike's. So at least its an option to not use the thermostat. I then put in a new thermostat but no improvement. Then proceeded, as planned, to clean out the Thermostat housing. By the way, the housing looked fine, some build up, but no blockage to harm water flow from a visual inspection or with a water hose. Took out the sensors first and then soaked the housing for 1 hour in Acid. Please be very careful if you try this( eye & skin protection please). Also I would recomend to block the 4 ports and pour acid in until full,rather than submerging the entire part as I did. (The acid takes the outer paint off as well). This made a dramatic improvement. Starboard riser (not port) will get hot to touch, but the thermostat would kick in and got very cool....even at 2100 rpms. This was a dramatic improvement,since before it never cooled at all. Took the boat out for test drive, but at higher 3000 - 4000 RPMs, strboard got hot and did not cool fast enough. Port riser stays ice cold throughout and Engine stayed at 170-175 degrees. I have ordered a new housing , to see if this helps, but I am not optimistic, since Mike also tried this. Mike. did you try Mercruiser original made housing..it should say ALPHA on top of it?? I want to try only original parts, not aftermarket, becuase I think this is the cause of the problem and would like to rule it out. My risers and manifold are AP(Aqua Performance) not Mercrusier OEMs. I think this engine type maybe very sensitive to changes as you and others have suggested. Even though after market parts are OK,there maybe cases where slight deviation cause flow problems.! Next steps: I am trying a lower range thermostat .. which seems to keep my engine cooler 165 degrees (not 170-175)at dock and also risers cooler...but have not had time to take it for a test spin at higher rpms. 2) rig up a set up to transfer a small amount of water directly from the incoming hose to strboard riser...with the rest going to housing as usual. 3)do as Guy suggested above."