I had the exact same problem. People thought I was crazy, but when I read the temp. with a temp. gun, there was about a 50 or 60 degree difference. Like yours, the starboard side manifold and riser was so hot, I couldn't put my hand on it, but the port side, although hot, was not too hot for me to put my hand on it. I completely dissasembled eveything from the thermostat (replaced that) all the way to the shutters. I cleaned everything out, ran a hose through it then reassembled it. I still had the same problem. My next step was to check the compression, but I had the marina mechanic look at it, finally. He said that is normal. I believe there is something called a reclaimer on the starboard side of the exhaust, which reburns or recirculates the exhaust before it exits, for epa purposes. Anyway, I'm glad to see it seems to be a normal thing for those engines.