Before you pull apart do a compression test all cylinders - that may highlight a problem on over heating side?  Looking at pictures you probably have a buildup in cooling galleries of salt on that side of motor - strip it down and clean out as much as you can?  Talk to an experienced Yamaha shop to get some pointers (and a quote )?

thanks for the ideas, much appreciated. I’ve given it a good clean out. Yeah I should have checked the compression first, I do have some reasonably recent numbers and I’ll test it when it’s back togeather.

I’ve swapped the thermostats so if one is bad I should see the problem on the other side at least. But i tester both in a pot and they both opened at 50c.

I’ll check the tolerances on the head before I put it back and a new gasket.

I’ll let you know how it goes when I test it but I’m thinking it’s something other than the galleries I can see

I have messed with non outboard engines for near on 1/2 century.. the basics are all the same..
 Motorheads suggestions are on to it,  trick of the trade, specific experience stuff...


I’ll check the tolerances on the head before I put it back and a new gasket.
 We used to do that, decades ago, and got burnt too many times.. regardless we have always skimmed a head since never had an issue 5 or 15000 miles down the line.


I’ve swapped the thermostats so if one is bad I should see the problem on the other side at least.

 According to johnson and couple other brand service/ repair manuals, new thermostats  are part of service.
As routine on a engine , be it a SB chev or a classic british whatever , a new thermostat regardless if checked out was always done... AND the new one always checked in the  electric jug and thermometer.
 One learns very early on that these little things, including water pump and oil pumps, cost a little more but end of the day one life becomes unstressful.
Or put it another way.. Murphy is you best friend, listen to his laws and avoid his consequences
 just a couple thioughts from a grumpy old man who hasnt gone grey yet

Poppet valves?
My honda has valves that open over a certain rpm, pressure driven, and they allow extra cooling water around the cylinders at high rpm.
My old honda had a rubber bit, that got salt under it, and IIt would clamp shut on the plastic valve and stop it opening.