Re: Johns 70 VRO overheats in slow speed
I had a Merc that did the same thing (newly purchased, used engine, first time out). Later, after investigating I found out that it had a stat and a bypass valve (somewhat larger hole). At low speed the stat was in control. At higher speeds water pressure (from the stat or driven into the water pickup on the lower unit by boat movement) forced the bypass valve open.
Both are located on top of the water jacket that covers the head (3 cyl) under a common cover.
The pee was always present as it came from the exhaust manifold where water is not controlled by either valve and is present all the time. You can look at yours and see if you have the same setup.....probably so.
So, like you, at slow speed I got the horn on constantly but all I had to do is speed up and it went away. Slowing down after a few minutes it came back.
So #1 on tas's list (the stat) is probably your problem. You can take it out and test it on the stove in a slowly heating pan of water using a candy thermometer or something that will record the temp. Opening temp will probably be somewhere between 120 and 145F.....temp is stamped on the pellet part of the stat and it is the temp where it starts to open. Will take 15 to 20 degrees more heat to get her fully open.
If you boil the water for say 5 minutes and it is still shut, you have your baby.
My over temp sensor is set to 195 and since water boils at 212F STP, there's your bounds.
HTH
Mark
