Sometimes 28 degrees isn't enough. Let me say it a different way. 28 degrees is enough at 3000 rpm. But the advance isn't enough at 1500 rpm.
Try 32 at 3000, it might make the difference at 1500 during the transition. This problem is also known as the dreaded "009 flat spot."
Also, when doing this you must slowly advance the throttle. Super slow, or you'll pump the accelerator jets, which is a separate issue.