Honda 150 Heat Warning

Question hoping to get an idea where to start.

I have a 2004 Honda 150 that after running around 5 minutes at 4k-5k rpm the temperature alarm sounds. The motor is not hot and if I turn the key off and back on its good for another 5 minutes or so at the same rpm range.

I can run all day at 1k-3k and no alarm.

I bought this motor used and the alarm has gone off since I installed it. Before I ran it I a did maintenance and changed impeller, thermostat, oil, high and low pressure fuel filters, plugs.

I did notice that my water separator on the motor does not have any sensor leads on it and I thought I recalled seeing that on another BF150. May not be related but adding that note anyway.

Any input would be appreciated.

Thanks

Gene

Take it in for  the engine to be clean out the water jacket .There is to small of wholes and they lime up slow water flow. If you do not understand  then Contact       

 http://www.harborrec.com/4-2/ 

  They help me with mine .

impeller blade not clock right ???

A couple of things...

When the motor is running have you verified water flow via the tell tale stream or at the rear of the motor (outlet of the thermostats)

Your sensor is located on the head, when the alarm goes off can you take an actual temperature reading of the head via an infrared thermometer or by a surface mount type thermocouple? From there you can compare the actual temperature to the rating on the over temp sensor. 

Lastly, interesting that the motor only alarms at a higher rpm.  At a higher RPM your motor requires more water to cool the engine.  Sight unseen, seems like a water flow problem.