I just replaced the thermostat on my 67 6hp evinrude. with the old thermostat, water would only come out of the exhaust/water hole on the backside of the engine at mid to full throtle. i removed the t-stat and ran it for a day, and water flowed out at all times, (reverse, idle, and forward). when i put the brand new t-stat in it does what it used to, and when i idle for wxtended periods, the enging gets very hot. the water pump was just replaces, and all cooling passages are clear and able to flow plenty of water. any ideas on why there is no flow at idle would be appreciated.