www.iFourWinns.com

Just to add more for future readers:
Clogged (rusted) exhaust manifold or clogged thermostat housing can also cause overheating...if water cannot get out of the engine, then new (cold) water cannot get in.

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"Knot Easy" 2000 Horizon 240 Volvo 5.7GS /SX
tow: 2017 Honda PILOT EXL-AWD
prev. boats:
'87 Chaparral 198CXL 4.3 OMC Cobra
'69 Jetstar 16ft Ski Boat, 115hp Yamaha
'68 Aluminum Jon Boat, 3hp Sears
'64 Water Wings

Right and if its clogged just on one side, all the water will go out the other side (path of least resistance). You will then have one riser at normal temps and the other hot enough to burn up the rubber exhaust hoses.
The ones on my old OMC were surprisingly heat resistant. When I had my bad overheat last summer, while I was waiting for the tow to come, I took out the IR heat gun and took readings on the one piece manifolds. 375* on the riser end!!! Yet when I took it all apart the hoses were not melted at all, a little stuck on the aluminum pipes between the 2 rubber hoses but otherwise looked fine. I replaced them because they were 25 years old and pretty stiff but still surprising. The engine was getting NO cooling water at all because that small hose from the transom mount to the P/S cooler popped off.

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88 Four Winns 200 Horizon
4.3 OMC Cobra-4bbl
2002 Walker Bay 10/2012 Suzuki 2.5
2008 Walker Bay 8

1998 Jeep Grand Cherokee 4.0/Selectrac
2007 Jeep Grand Cherokee 5.7 Hemi/Quadradrive II