I have an MD7A vintage 1977 with salt water cooling and began to get white smoke and overheating at RPM over 1000. Problem ultimately traced to partially clogged input to water pump. Removed right angle pipe fitting on starboard side of transmission and removed crud from inside transmission while blowing water through from water hose into port side of transmission. You should be able to pour water thru here freely. This solved the problem so that without thermosat in place the temperature never went off the bottom of the gage. Then replaced thermostat and got steady 140 degree temperature.