The elbow that connects the downstream side of the carb to the intake port on the engine is fitted with a recess and accompanying O-ring on the surface that mates with the port. If the retaining nuts on the elbow have been loosened and the elbow itself moved, it's real easy for the O-ring to either become misaligned or fall out altogether. If this is not detected during re-assembly, you'll wind up with a nasty vacuum leak and all the headaches that can accompany it.
Learned this one the hard way!
-Charlie
Too bad it was at your expense! 