[Profittakr]

'88 Yamaha 70etlg. Boat was running fine then it reduced power to about 1/2 rpms. After about 15 minutes of near full throttle run the rpms dropped to 1600 on my tach but not sure if that is accurate. Boat ran for another 15 minutes back to the dock at 1600 rpm smoothly and without issue but that was the max rpm it would do.

Some other info.
- I noticed during the run that my volt gauge was maxed out at over 16 volts.
-I never got a buzzer. I have since traced and tested horn and it works. Someone has been in there so maybe I fixed a loose connection.
- Oil tank was topped off the previous day and still plenty of oil in it.
- Did not notice if tell-tale was working during the reduced rpm session but it does work at home on the muffs once I had time to think about it.
- Since no buzzer I did not think to look at my multi-function guage to see if it indicated a problem. Why might the buzzer not be working?

I am tempted to put it back in the water with pre-mixed fuel to protect the engine that way and see what happens. If it goes to full throttle and then slows down like before, then I assume it would then be an overheat situation but guages might indicate otherwise. Thoughts?

Sorry for long post just wanted to get all the information out there.

[Profittakr]

Tested compression and was 110 - 115# consistently. Also tested thermostat and it began to open at 120 degrees; fully open by 140 degrees.

Slowly working through things but I am not even sure what system to be looking into at the moment.

[Profittakr]

Update: situation resolved. Turned out not to be an engine issure after all. The cover plate where the throttle cable enters the engine was cracked. The throttle cable would slowly slip out of place over time and the throttle cable would move and close the throttle. Cover was repaired and this elusive issue was solved. Just wanted to throw that out there in case others have similar issue. Lesson learned; don't always assume it is the engine. You need to look at all possibilities.