Not terrible familer with raw water cooled systems like that, but it certianly sounds like you have a restriction. The water pump's are enough to move it at high RPM but not at low.

I'd probably start at the drive and blow some water through all the passages to make sure they are clear. You guys have problems with Zebra mussels down their?


Their should be a valve in the system somewhere to prevent water from coming back up into the exhuast in a following sea. Some Mercruisers do it with flaps, I have seen those break off and cause havic.

I'd pull those balls, run it and see what it does.

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1969 280SE
2027 Ram 1500
2026 Genesis GV70
2007 Tiara 3200
1988 Egg Harbor 37
1981 Boston Whaler 17