Do either of you have an infra red thermometer? Something like one of those can tell you quickly if the motor really is over heating. Doesn't have to be an expensive exotic thermometer. I think I paid 15.00 for the one I bought ages ago.
Is your garden hose hooked to something that can deliver a LOT of water? In other words are you absolutely sure it could stay up with the demands of a motor turning 2000 rpm? If so, you can use an infra red thermometer pointed at several different places on the engine head (near the thermostat housing for instance) and block after the engine has been run for 3-4 minutes at 2k rpm. See if the thermostat housing and the area around the spark plugs start getting hot - like 180 or higher. If you see much over 200, stop the test right away. There is something wrong for sure!
If your hose isn't up for supplying the kind of volume the motor is going to need, or you aren't sure, you'll need to locate a tub, or maybe cut down a garbage can enough to get it to fit under the motor and perform the test that way.
I haven't had it happen to me (yet) but there are 2 relief valves that come into play when you're using the rinse function to rinse the motor. Not many use it as the muffs work just as well, and there's little chance one of these relief valves can stick using the muffs. Thought is, if they are gummed up/stuck, that might cause the engine to over heat under load. Check the relief valves out here, # 4, there are 2 of them.
