Hello all, need help with a 3.0l OMC sterndrive in a 28' Harris pontoon boat. Long time reader first time poster 
Compression at 120 across the board. Rebuilt engine (bought from retailer not self done), new manifold, new thermostat (3x, one $40 dollar mercruiser model), All new gaskets and seals, of course. Exhaust riser restrictor plate installed today as a last resort and still not warming up. New exhaust flapper with bushings as well. Engine temp at around 120 loaded with ten people and gear. Yes I checked the gauge and wiring, and yes I checked the sending unit. I can touch the exhaust manifold and riser and theyre cold under the carb, warm at best everywhere else. I have tried gaskets from pre-92 and after 92 for the thermostat housing and exhaust riser, as the diagrams I read show different gaskets for those years. The thermostat housing is hotter where the spring end goes in (bottom) than at the top like the thermostat(s) isnt (aren't) opening. My U-shaped hose is very hard, which I read may be exhaust gasses circulating back into the cooling system, however the temperature difference between the upper and lower thermostat housing leads me to believe these thermostats arent opening up for some reason. Sorry about the long post, just trying to give as much info as possible! Im at my wits end with this thing!! Please help!
Compression at 120 across the board. Rebuilt engine (bought from retailer not self done), new manifold, new thermostat (3x, one $40 dollar mercruiser model), All new gaskets and seals, of course. Exhaust riser restrictor plate installed today as a last resort and still not warming up. New exhaust flapper with bushings as well. Engine temp at around 120 loaded with ten people and gear. Yes I checked the gauge and wiring, and yes I checked the sending unit. I can touch the exhaust manifold and riser and theyre cold under the carb, warm at best everywhere else. I have tried gaskets from pre-92 and after 92 for the thermostat housing and exhaust riser, as the diagrams I read show different gaskets for those years. The thermostat housing is hotter where the spring end goes in (bottom) than at the top like the thermostat(s) isnt (aren't) opening. My U-shaped hose is very hard, which I read may be exhaust gasses circulating back into the cooling system, however the temperature difference between the upper and lower thermostat housing leads me to believe these thermostats arent opening up for some reason. Sorry about the long post, just trying to give as much info as possible! Im at my wits end with this thing!! Please help!