No thermostat?
 

I have a 2007 yamaha 70hp. 230 hrs. 2-3 years ago I had to pull the head and clean the water jacket in the head and replace the thermostat because the jacket was completely clogged. I checked today and the thermostat was gunk'ed up and rusted and the housing for it in the block was gunking up again. I cleaned it up and ran it on hose with no thermostat and it has a lot more flow. The thermostat opens supposedly at 120 degrees. I'm thinking if I run without the thermostat with more flow I won't get all this build up in the water jacket. I never run more than 4 miles one way and figure maybe put the thermostat back in the late winter when the water temp. really drops. Anyone running without a thermostat ??? Anyone know if it will hurt anything to run without one??? I know of 3 other 3 cylinder yamaha's that had this happen so it must occur often and all I can see is because of a lack of water flow thru the thermostat.

I don't think the thermostat opens at 120 F. I believe most open at 190 F.

I never ran a engine without the thermostat. I have had to clean out small sticks and leaves, but not gunk'ed up.

After washing the thermostat with a hose to remove the small stick and leaves, I would boil the thermostat on in a pan on the stove. You would be surprised how much more stuff comes off the thermostat when it "opens up". I did not take a picture after boiling the thermostat.

I say 120 because it is stamped 50 degrees centigrade...Yours looks like mostly trash . Mine is mostly dirt like sand when it dries like it's not flowing thru and caking up. Pic is not very good and most actually crumbled off .

I've been running without one for years. I think if you were in a cold climate it matters but not around here.

I ran friday 3 1/2 miles one way and back to go crab and we caught but 1 crab and the only thing I noticed was less pressure out the pee hole and more out the exaust below the cowling.