It overheats when under power. At home running on a water hose temps run 145 degrees at idle. Here's what I've done since early last year: freshwater impellor ( impellor was fine, pliable not broken), engine water pump, cleaned heat exchanger for water cooling, 145 degree thermostat both manifolds and risers (port manifold rusted thru at #3 exhaust - not into water jacket but into bilge. Stbd manifold had evidence of water leaking into exhaust ports - presumably from manifold itself). Here is the wierd part; after replacing the port manifold we filled the heat exchanger with water and a weird thick black soap bubble like stuff came bubbling out so we tried just flushing it out (just running overflowing water in the heat exchanger) with eng running and the water was black. We also found the choke hadn't been working so fixed that. I am beginning to wonder if the block has soot in it from running rich for who knows how long and that's impeding the heat transfer in the engine. We ran the boat for a couple of years with no problem and don't think there are any impellor parts in the cooling system. How can I test the raw water volume at idle? Should it fill a 5 gallon bucket in 30 seconds? Any help is appreciated, the port engine runs at 145 to 165. I'm beginning to think I need to dismantle my raw water cooling system and inspect and flush. Does that require pulling manifolds? They aren't too bad now, I broke 3 bolts on the port side but was able to get them out.