Low speed needle too rich. I use a fairly reliable method of getting the low speed correct. I set the top end to peak and leave it there temporarily, then procede to lean the low speed needle 1/8 turn at a time testing the transition and idle at every change and re-checking the top every other change. When satisfied with the idle and transition richen the top down a couple of hundred rpm from peak. Be willing to sacrifice a little idle smoothness for a better transition if necessary.