68 Johnson 9.5hp, What is water bypass plate for?

I'm wondering what the water bypass plate is for. I only know that's its name because I found it on a parts diagram. Now "water bypass" suggests that this is where I can place a motor flusher for running on land but I tried that and had to quickly shut it down because there was no water coming from the pisser. So if thats the case then what is it for?? 

When I tried the flusher, I didn't have the plate off, should I have removed it for it to work? The plate has holes in it so I didn't think it was necessary. But after opening it up, I can see there is a hole on the right that leads to the water intake. And there appears to be a hole at the top left that I assume leads to the water pump. 

So if this is not intended for motor flusher use then what is it for? And if it is for motor flusher use, then how do I make it work?


Also, when I was running it in a barrel once, I allowed the water level to go down until it passed this plate and the pisser ran dry so I shut it off. Seems like this indicates 2 water intakes 

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1 Answer

  • 2 months ago

    The water bypass channel allows cooling water from the impeller assembly to bypass the power head assembly.

    That’s necessary as there should be a thermostat (can be missing on older engines due to bodged fixes) and the thermostat should only open and allow cooling water into the galleries of the power head once it has reached normal operating temperature. No thermostat hugely increases engine wear because a cold engine is a tight-running engine, and also increases emissions and fuel consumption.

    Your bypass cover plate will have two main purposes: it allows cooling water to bypass the power head when the thermostat is closed, and it also allows blockages to Be cleared. A third reason it’s there is that it is often easier to manufacture the engine with one side of the bypass channel missing and to then use a cover plate afterwards to close the open exposed side of the channel.

    A coolant flush of the head requires the ‘stat to be removed. Whilst the ‘stat is accessible consider replacing it as they’re cheap and don’t last forever. If yours appears OK  and is not all caked with corrosion and muck then you should be able to see a two digit number stamped upon it. That’s the opening temperature for that unit and by suspending it from a piece of string/wire into a pot of very hot water with an accurate measurement thermostat such as those used in kitchens you can add boiling water or cold water to check what temperature the ‘stat actually opens at. It should be a not more than a degree or two different to the marked temperature on the ‘stat. If you do change the ‘stat for a new one then still test it as new units can definitely be faulty as well. 

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