1969 DT115 2cycle slows, misses, alarm

[crestlyn]

My engine idles great, runs exceptionaly well, water is cool coming out of
engine, yet! after a short run about 10 minutes it gives an alarm and begins to bog down and miss.. cylinders are cool...top of engine at timing
indicator and top nut is hot. after a short time of idle about a minute it runs very well again..Help!!

[boats.net]

Probably need to replace water pump, check thermostats.