If you still have a thermostat installed, you should not close the bypass all-the-way. With the bypass closed, and the thermostat closed (as it would be when starting cold), there will be no water flow at all until the engine warms up. This can quickly overheat and damage the lining of the exhaust hose, and even melt a plastic waterlift muffler if you have one.
It's the reason the bypass exists in the first place.
It doesn't take much, but you must have some water flowing through the exhaust system to prevent damage. Years ago, Hanley did some experiments on reducing the exhaust waterflow, but I don't have the link handy.
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@(^.^)@ Ed
1977 Pearson P-323 "Dolce Vita"
with rebuilt Atomic-4
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