My sons KX85 did the very same thing, and I could never jet it out. Finally one day a main bearing seized, we figured it must have been tight from day one and kept it from reving fast from an idle or low speed. Once you got rolling it ran great, or if you rolled into it easy it ran great, but if you wacked it open from idle or a low speed it would go into a terrible bog.
I am not saying this is your problem for sure, but that is what I ran in to on my sons 2005 KX85.