Too rich

Further to the "Too Rich" and Sooty" condition".  When you "rebuilt" the carbs, did you remove the "Coaster Enricher piston/diaphragm"  If the answer is NO....given the bikes age, then there is the real possibility that the Coater Enrichment Pistons are all "gummed-up" and stuck in the closed position. (This cuts off the supplement mixing-air circuit)   That will  cause your "rich" mixture.  Leave the "guts" out and merely replace the cover along with a pliable one piece/solid gasket. to seal this system.  The  Enricher System is not necessary or required and will have no adverse affect. This, along with several other simple mods will increase your fuel mileage with no detriment to performance.

I haven't touched the carburetors yet, except for the needle and float chamber gaskets, which have been replaced. Do you mean the cover with three screws on the sides of each carburetor?

My intention was to get all the spare parts, starting with the brass floats, and then later disassemble the carburetors. In the meantime, I wanted to use the bike a bit to see if there were any other obvious problems.

 

Do you mean the cover with three screws on the sides of each carburetor?

Yes