wow this is amazing, i have had the same problem exactly, on a 1991 uk sumo 1.2
i have been trying for months to fis it, 2 experts have told me its the carb thats at fault, but reading ur post i am not sure now. hte newest clue i have to solving the problem is that on mine, the bob weights in the distrib is fauty.
if u do get an answer please email me asap, and if i sort the problem i will do the same.
also if you have a working complete carb would you be interested in selling it, i am desprate for one at the mo, my one has been broken beyond repair.
also i herd that sometimes the jet at the bottom of the float chamber wears funny, and so allows to much fuel through.
although i should add that this did not solve the problem for me. happy drivn dude.