Hi Matthew, I have owned and worked on these engines with a similar problem. Some of the answers have addressed some of the problems. before coming to the most important issue I agree with the cleaning
out with CLR or similar. Salt
Away and various products can be used. An 'acid wash' was a term used in some trades previously. A soaking of all the internals for several days may be needed to shift some of the more dense build up. The solution must not back flood through the exhaust pipe and my suggestion is that you remove the water injected exhaust bend and thoroughly clean it out using mechanical and chemical means. These do block up and can also suffer from internal cracking. See. https://www.marinepartseurope.com/en...-26-15108.aspx
With the water injection hose lifted to a level to allow the CLR or similar fill all the cavities the water chambers can be left soaking. An initial week or so of soaking, after which a good run with normal cooling water will usually give a lot of 'crud' coming out the exhaust.
However to the often crux of the matter. Bearing in mind that some small cavities may be almost impossible to clear without mechanical means it can only be hoped that they are porous enough in the build up to gradually clear.
As mentioned, the crux of the matter has been in some cases the actual thermostat itself. The temperature of thermostats in these engines start to open (from memory) at 58 degrees Celsius. Putting in a regualr automotive thermostat opening at even 80 degrees Celsius would encourage deposits.
After a certain temperature of heating
, salt water
'drops' the salt out of the water and it deposits itself on the heated surfaces of the internal chambers in the engine. When testing the thermostat it must start to open at the low temperature or it is faulty.
When replacing these particular thermostats with the genuine part, I would always drill a one eighth hole in the opening cap plate. That assisted in eliminating air bubbles but more importantly allowed a very small flow of water to gradually equalize the water temperature each side of the thermostat cap and encouraged a small water flow so through the chamber which helped relieve a sharp transition from cool to hot as the thermostat started to open.
As a side benefit, if the thermostat failed to open through a fault, which these thermostats did seem to suffer from after some time, the small one eighth hole ameliorated the effects of a jammed thermostat.
To pump through the CLR or similar, removal
of the water inlet hose after shutting off the seacock and putting that inlet hose in to a bucket of the de-clogging mixture will take it through the engine chambers. After the initial flush through the whole system, when it is observed that the solution is coming out the exhaust pipe there is the option of removing the hose that injects water into the exhaust elbow
and equalizing the height of the inlet hose and that exhaust water injection hose so each may be topped up until full, above the engine water chambers.
There may be air pockets, bits or impeller vanes and suchlike in the system so it is a matter of checking each part out, including de-laminating hoses and I have probably overlooked other possible issues here. For actual cleaning of systems I have sometimes chosen to take the thermostat out just to allow a less obstructed flow of the cleaning solution. Most important is the genuine low temperature opening thermostat and my personal modification of the one eight hole to assist with future temperature transition. Checking the impeller and the impeller cam may be worth while of course. If I have made any errors in the above or omitted anything , then others may be able to improve on suggestions. All the best. Regards, John