I have exactly the same lawnmower and exactly the same problem. I have dismantled & cleaned the carburettor but this has not helped, although it has cured a rough idling problem. I too have suspected something is binding as when I tip the mower on its side,(WITH THE ENGINE STOPPED) engage the blade and then turn it by hand there seems to be undue resistance, although it appears as if the blade brake pads are clear.(BETTER TO DO THIS WITH THE SPARK PLUG REMOVED). Hope this helps.
Harvey A
May 2007