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70 hp Johnson Overheating

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pgorrell

Member
I have a 70 hp Johnson that I put on a 20 pontoon. When operating for long periods of time at full throttle, the overheat horn will go off. If I slow back to an idle it will turn off in just a few seconds. I have replaced the water pump and the thermostat has been removed by previous owner but I have a new one to put into place. I was told by the marine shop that removing the thermostat would not make any difference except in cold water because the engine would never heat up enough to run smoothly.
 
racerone said:
Model # is ?-----Perhaps motor is mounted too high.
Click to expand...
It is a 1988 on a Beachcomber 20 ft. Pontoon. If I mount the motor in lower, the motor will almost be setting in the water below the Exhaust/Water Ports on the rear of the motor. I am wonder if the transom of the boat is diverting too much water away from the motor.
 
The water line at rest or low speed should be above the anti cavitation surface above the prop and therefore above the water pump. A pontoon has no hull surface in front of the motor as a guide to minimum water level. You need to observe how high the motor is lifting the pontoon and see that adequate water is available there at the water intake holes. It would normally begin cavitating long before the water reached the danger point. I think the problem is elsewhere. Has this motor been used in saltwater?
 
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A thermostat is needed regardless if it's cold or warm water.

Pontoons can be tricky because they create a lot of turbulent water around the transom. if you're getting a lot of air bubbles around the lower unit it will causing poor water flow up into the block.

Is this motor with power tilt and trim? What is your trim angle when this happens? Maybe trimming down will help.

Wasn't there a certain model(s) of OMC 70HP engines that had a factory TSB for cooling mods? Or, was that just routing of the telltale/water pressure tube?
 
Thanks Kevin, I have done these myself. It is beyond me why the engineers didn't come off the top to begin with. Its a great way to purge the cooling chamber of air, especially at idle and low speeds. At WOT, however, it may not be the problem unless the water inlet is in turbulent or aireated water. A pontoon may create just that too.
 
I have done the update, my fear is too much turbulent water around the intakes of the motor. I was told if I put a stabilizer on the motor it would help with the turbulent water. I have also thought about reducing my center transom in size.
 
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