Was it overheating before you got it? Is the head/block discolored/burned paint? You won’t see any corrosion coming out of the water tube when probed that way, it is the actual smushed rubber of the water tube that may be blocking water flow.
OK, so the water pump parts are Sierra, you may want to invest in an OEM water pump kit just to rule out that possibility. We will see what happens when you replace the thermostat. Remove the overboard indicator plastic pan restrictor fitting just in case there is crud in there blocking the hose. Again, be sure the water tube isn’t bent and is fitting correctly into the impeller housing grommet.
Post some pictures of the powerhead so we can see if overheating was an issue before you got this engine.

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Rusted Junk

OK, so no real big evidence of overheating before you got it… Did it overheat as soon as you ran it the first time? Or, did you judge cooling by the lack of telltale spray? Again, those telltales do get plugged up. It is best to remove the plastic nipple, then blow back through the hose while the engine is running. Once the hose is clear, let it run with the plastic nipple for a few minutes to clear any other crud that might get stuck in there, then reinstall the nipple. The fact that the engine doesn’t show much signs of overheating prior to you getting it kinda suggests the grommets might be OK…The usual scenario is that these engines get overheated, then folks try everything to fix the problem except the grommets…The engine continues to overheat, then the owner gets rid of the engine….But, usually there are obvious signs of overheating, burned paint, melted wiring….
I know we have focused on the flaws of these engines, but don’t misunderstand. The powerheads and gearcase are really very rugged, and they are great little engines for the most part. A 15hp short shaft model is pretty valuable these days, so don’t “give it away”….Who knows, you may want to keep it…

• This reply was modified 2 years, 11 months ago by fleetwin. Reason: correction

Cool!

Bill is right on….Those darn crushed grommets may show water flow, but it is not enough….The classic symptom is starting the engine, seeing water flow until it starts to overheat, then it will suddenly blast steam out of the telltale and really heat up. Keep in mind, some of these engines have two of these pesky grommets, if it has the open inner exhaust tube. I surely don’t want you to rip this thing apart for nothing, but the grommet(s) will have to be addressed at some point if cooling issues can not be resolved with simpler inspections/repairs. The grommet/water tube set up was improved over the years, but even some of the “improved” set ups get crushed as well. Needless to say, this is more of a salt water issue, but does happen on fresh water engines also.
And, like Bill says, running an engine on “muffs”, means very little. Other than “it runs”. Keep in mind that the high pressure from a garden hose can be enough to overcome restrictions posed by some crushed grommets….D

Yes, way too expensive….See my current dockside chat posting about how to make those gear cogs mesh a little tighter….Keep in mind that this tiller throttle should only be operated when the tiller handle is fully extended in the horizontal position….The gears will get messed up again if the throttle is operated with the handle in a vertical position….An engineering flaw indeed, but not much that can be done about it….