Carb Problems

Hello; I have a 1982 KZ440A. I have had the carbs rebuilt recently. When I accelerate the engine and then let off the throttle, it takes an abnormal amount of time for the engine to return to idle. I have checked the carb linkage, the throttle cable and hand throttle and they are all moving freely and they return to idle position properly. What could the issue be? Thanks, skyrabian

Air/vacuum leak. Most likely the rubber carb and/or airbox boots.