too rich of an idle mixture. when you throttle down the engine it is still clear of excess fuel, but as it stays longer in idle, its loading up from too much fuel and slows down. Do a pinch test on it, run it at high rpm then bring it to idle, let the idle settle to a steady RPM then pinch your fuel line. if you rpms quickly raise then drop off, its good, but if they raise more then 200 - 300 rpm your running too rich. lean it out till you get a slight rpm increase and then it cuts off. good luck. BTW i used this method on a newly broken in motor on one of my students trainers and we got the engine dialed in nicely. It also works for the high end needle too. same method, just at full throttle instead of idle, but i use a tach instead of the pinch test for high rpm.

sean