That's all very good advice from Martin. Let me add a bit more detail. My usual protocol is start with the simple, then move on by level of difficulty. On this particular engine, the t-stats are the usual culprit, so that is the first place I would look.
If you have a temp gun or other way of measuring engine temp, warm up the engine and test the head temp. This engine does not reach full operating temp at idle, so you need to run it around a bit before taking your readings. Operating temp is around 160 - 170 F. If you are getting a full 160 F on BOTH heads, then look elsewhere for the problem. While doing this, test the temp of the water coming out of the indicator. It should be about the same temperature as you would feel in a hot tub.
If running cold, pull the t-stats and first check that neither is stuck open. If so, replace both. If not stuck open, heat them in a pot of water. They should begin to open at 140 F and be completely open at 160 F. Completely open means at least 3 MM. If either fails the test, replacee both.
If operating temp is not your problem, then as Martin said, decarbonize the engine using either Sea Foam (cheaper) or YamaLube Ring Free (more expensive, put better.) Caution, make sure you decarbonize in a place where all the dirty crap that comes out of the exhaust can be dissipated. I still have black splotches on my garage door from decarbonizing an old Johnson SeaPro.