Engine shuts down

[kykeka]

1990 23' Pro Line/Cuddy, 1989 Yamaha 225 Excel,disconnected oil pump,mix 50 to 1 oil, boat runs all day under 4000rpm, if I go over 4000rpms alarm sounds boat shuts down into a safe mode.I shut off motor, start up then go in reverse then forward then alarm shuts off.I can run all day under 4000rpm, about 30-35mph. I go off shore about 27-30 miles in the gulf.New water pump,new thermostats installed no good ,sounds to me some kind of sensor, mechanic disconnected sensors he said, but I still have this problem. Thanks Frustrated Mark