Having the engine bogg out when applying throttle (choke down) as you state usually indicates that at least one of the carburetors is fouled somewhat.
The small vertical off center brass tubes inside the larger vertical high speed nozzles are known to become restricted, and also the brass high speed jets that are located horizontally in the bottom center portion of the float chambers clog simply due to sitting... either of these areas could cause your problem.
Another possible cause... the carburetor throttle cam roller should be approximately 3/8" in diameter. Some of the later model rollers rather than being just a one piece roller have been upgraded to a two piece roller... a smaller diameter black roller covered by a clear sleeve... the sleeve has been know to break away resulting in a roller too small which throws off the throttle timing in relation to the actual engine timing.
Check the size of the roller to make sure it's approximately 3/8" in diameter.
The throttle butterflies should just start to open when the scribe mark on the metal cam is dead center with the 3/8" diameter roller... not before or after.
