Help, Evinrude Fleetwin engine problems
 

I just assembled a basketcase 1950s blue Fleetwin 7.5hp and took it out to the
lake for a checkride. It eventually started, and took off pretty good, but
noticed the needle valves didnt seem to have much of an effect even when
screwed in all the way. Richening the mixture would make it run rough though. I
had to keep it above 50% throttle or it would cut off. It was pumping water,
and pressurized the gas tank and the cylinders were hot but no too much so I
guess. After a few minutes, it started running rough and slower and slower even
at 100% throttle. I shut it off, and noticed it stopped quick. It was hard to
turn over by hand but not too much so, and made a squeaking noise. The next
day, I took off the covers on each side of the cylinders and the pistons do not
seem scored. It turns over normally and has great compression. The fuel mixture
was 24:1. The head bolts are very tight, so I'm not removing them for now.
Looking in the plug holes shows some carbon buildup near the tops of the
cylinders as least as far as I can see. Any ideas what's going on? I cleaned
the carb pretty good before I assembled it.