273 Engine Help

That, to me, sounds like it's flooding or at least running too rich. Try and figure out if it's running too rich by going back to the tailpipe and looking for black smoke or "chugging" while you're trying to rev it out. Maybe the float level is too high, or there's dirt in the needle/seat, or the metering rods are stuck in the "up" position. If it only does it when it's hot.....to me, that's kind of a tipoff that it's too rich, however, my buddy's corvair used to crap out when it was warm and it turned out to be a bad Pertronix unit. Good luck!