I have been working on this issue since last fall. Now it's getting warm enough to start again. Not sure what happened but at some point while tinkering with my motor, mariner 200HP EFI OG 571537, I lost some or all of the cooling water flow through the starboard head. While running the engine, the water flow out of the pitot tube has diminished considerably and while the port head is warm while the engine is running the starboard head gets quite warm- almost too warm to hold your hand on it. Changed the water pump. Changed the poppit valve. Pulled the heads to check for any unseen obstruction. Pulled off ALL hoses and checked for obstructions. Flushed the heads with the thermostats off. Good flow there. Took off the lower unit and hooked a garden hose to the water pump tube and out through the thermostat ports. Good flow there, probably because of the increased water pressure. One thing I did was to remove the starboard thermostat and reconnect the cover on the head. Got good water flow through the pitot tube then. Put the thermostat back in and flow quit. Assuming that the water pressure was too low to have a good flow past the starboard thermostat. Replaced both thermostats after checking them for operation in hot water. Both opened around 143 degrees. I know there is water going through the engine because ample water runs out above the anti-cavitation plate. My concern is that there is an obstruction somewhere in the water path through the starboard head. Any ideas on where else I could look or any other flushing method I could use?
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