overheating

[kem903]

i have a 1986 mercury blackmax. temp gauge shows it getting hot. water seeems to be flowing fine. gas mixture at 50:1 like the dealer said. the temp starts climbing at about half throttle. at full throttle it wil climb all the way over. let out and it will cool off to about three quarters. gauge doent have any reference numbers just five lines and a needle. any sugestions?

[boats.net]

Need to have engine temp checked to see if it is overheating, if not not need to test temp sender for temp gauge.