first never assume that the money you spent on the carbs covered everything. Ok you say it it not the carbs so i assume you have checked to make sure there are no vacum leaks that all gaskets around the carbs are sealing properly, the mounting boots have no cracks that leak and the vacum lines are all new with no cracks that can leak and the air filter will flow enough air for the bike to run correctly. The next step is to check the plugs even new plugh can have defects that allow breakdown at a certan load. then check the level of fulid in the battery I know this sounds crazy but my 76Z1 would not run properly if the battery was low on water. seems the electronic ing. is a might senisitive to voltage flow. then ckeck and clean the electrical connections to the coils. You have already checked to make sure they are not cracked havent you?