[Dead Parrot]

I would think that high HC and CO might point to a too rich mix from the carb. Dark plugs and the sometimes black smoke also suggest a too rich mix.

Another thing to check is the exhaust riser valve. One stuck shut or mostly stuck will cause a lack of high end performance plus can damage the carb that also results in a too rich mix. If it is stuck shut, most of that side's exhaust gas is forced through the small intake crossover which both limits flow and overheats the carb.