Don T: I didn't mean to confuse you with "T" and "S" marks, I was just trying to keep you from disassembling the engine unnecessarily. Gerry Ide has a helpful suggestion about confirming valve timing. I'm not sure if the "S" and "T" are marked on the crank or the cam; I was assuming they were marks on the cam, but if the crank is marked, then I suppose you would have to open up the crankcase to confirm valve timing.
Everyone seems to think that the problem is NOT valve timing, I was just trying to come up with a way to confirm the mechanics short of opening up the crank case.
Thinking out loud, if the marks are on the crank (which makes sense, really), then one would need a degree wheel and the specification for when in the 180 degrees of rotation Before or After TDC the valves open to see if the valve timing is correct.
Again, if the problem IS the ignition timing (not the valve timing) or fuel related, then the mechanical valve timing isn't the issue.
I kind of agree with everyone else, the problem is most likely ignition timing or a fuel delivery issue (including the carburetor). At this point, you're probably better off sleeping on it, and tomorrow everything will be clear.
I only tried to offer a way to eliminate or isolate a valve timing problem short of pulling the engine and disassembling it.