1990 gsx750f wont idle properly
 

Can anyone suggest a cure for my problem bike which wont idle properly, I have balanced the carbs which are now in perfect sync, (using 4 pot carb balancer) but I cant get the idle to come down to 1200 rpm but remains at around 4000. the idle screw is not even touching the carb idle setter thingy, but if i force down the carbe idle body thing the revs will fall but as soon as the throttle is blipped the idle rises to 4000 and wont come back down on it's own without forcing the idle body thingy down. The carbs have already been stripped and cleaned no tears in the diaphrams were found, but it's entirely possible i've mucked something up, during the rebuild.
P.S bike runs and pulls like formula 1 though.