Excalibur29

I have a problem with my 115HP motor over heating.

I have done the following to try to rectify the problem.
Replaced water pump. thermostats, removed heads to check the rubber plug water flow control positions & condition (all OK) and replaced the head gaskets. The original gaskets were OK and there was no evidence of leaks.
Checked cylinder pressures all 120 psi when hot. Also checked timing (OK)

Engine runs perfectly until the thermostats open at 143 ...