mine runs 10min easy with a 2096/41033 with 6mm restrictor sidewinder pro blend 30%on a fairly large open track.
just to be sure your carb gap is about half mm, your total throw for the carb is slightly smaller than said restrictor and you have a proper race tune on the lowend?
a difference in 3 minutes is pretty large are you dragging any bearings? i run the tki4 and its not really known for good milage