brainpicker for the day, 1977 55 HP Evinrude cooling puzzle

[boltbreaker]

1977 Evinrude 55EL77D.Motor seems to "quit pumping" water at high speeds. This engine does not have a "tell-tale". It just shoots water out the back of the exhaust housing.Fact 1: The motor pumps GREAT water in a test tank and at low speeds.Fact 2:The motor is mounted on a boat to where the anti-caviation plate is about 1 inch ABOVE the bottom of the boat.Fact 3. NO WATER comes out the exhaust housing when the motor is out on plane around 3/4 throttle.I've heard that these engine will not blow water out the back(like it does in a tank or at low speeds) after the engine reaches a certain RPM. I have a hard time believing this. Maybe this is true? I've read on some of the other posts here that there is a problem with the impeller housings drawing exhaust gasses at high RPM's causing the engine not to pump water. Is there anything else that can cause this or is the motor just mounted to high?Anyone know how to test to see if the thermostat is working properly?Thanks for letting me pick your brain! -B-

[WillyBWright]

Is your Hot Horn working? Find the sensor on the head and ground it with the key on (running or not). If your not getting a Hot horn and it is functional, then your pump may be working fine. With the prop spinning and the boat on plane, most of the exhaust is sucked out of the prop. That would include expelled water. However I would hope to see something come out of there regardless, planing or not.You can put your own telltale on it. There is probably an 1/8" pipe plug somewhere on the head. Put a fitting in there instead and route a chunk of speedometer or aquarium hose out a hole you drill in a back corner of the pan.

[rayjay]

Get two heat sensitive "crayons" available at most welding supply shops. 125 and 163 F. At less than 1000 rpm after several minutes of running touch both crayons to the head, if the 125 melts and the 163 does not then the temp is right.After several minutes running at 4000 rpm touch the 125 crayon to the head , if it does not melt then the temp is correct. These tests need to be done in a lake I would think.These motors have a thermostat that allows the motor to run hotter at low rpm and a pressure valve that allows the motor to run cooler at high rpm. An infrared heat sensor pyrometer would probably also suffice. They hadn't been invented in 77 when my manual was published.Factory manuals show up on the web all the time.

[Joey One]

If your thermostat is easy to get to (as mine is),put it is a saucepan with water and heat it on your stove. If it opens before the water boils, the thermostat is OK. If its real cruddy from salt water, soak it is vinegar for a couple of hours andclean it with a toothbrush. Also check that the vent, a tiny hole or passage, is clean and open; so the cooling water can reach the thermostat.

[Walker]

To do the boiling water/pot test you need to do it correctly. You need a thermometer that will read high numbers and chect the water temp when the thermostat opens. The boiling point of water at sealevel is 212 degrees f. Your thermostat needs to open at somewhere around 140. Shop manual will give you the thermostat range exactly.