alarm sounding from throttle box

hi guys. i have an 80hp yamaha which starts on the button first time. after running the engine for a few minutes an alarm sounds from the throttle box and then limits the engine to about 1500rpm(like built in safety mode) engine is pumping plenty of water so not overheating. please please any ideas. many thanks. nik

Nik,
what does the alarm sound like, we need to know so we can tell you which alarm is going off.

its just like a constant long beep. someone has told me it could be the impellor

constant long beep is the overheat alarm. you will need to check the system.

I had the same issue with my 1998 C150 last fall. I had replaced the water pump impellar, thermostats, bypass valve and hose and tested the 2 temperature switches to confirm they weren't faulty. The problem continued. I removed the engine head covers and discovered the water jacket ports were plugged with silt and salt buildup. I removed the heads and mechanically cleaned them and then chemically soaked them as well as inspecting the engine block for buildup. I reassembled the heads with new head gaskets...no more alarm problems.

I had a good flow from the cooling water pilot hole, but that is deceiving....that's not necessarily an indication the water is flowing thru all ports in the water jacket. The top of the heads were not getting a good flow of cooling water. This engine has been operated for 12 years in the Galveston Bay, was always flushed and chemically flushed after each use, but the silt and salt buildup still occured.