It's been many years since my Suzuki GS750 days, so please forgive me if the following has no bearing on your carbs. If the carbs have fuel bowls, they will have a float and a needle valve to regulate flow. Excess flow would overflow through the tubes. If this is the case for your carbs, have you checked the floats and valves? Just a tiny bit of crud on the valve will cause it to stay slightly open. Also, your floats need to float in gasolene.... Again, I do not recall whether this even applies to the mikuni or mikuni style carbs of the day.