Been a looong time since I've done this in anything, and longer still for an Airtourer, but I vaguely remember the speed somewhere south of 60 over the top at zero G. Try a less savage pull on entry, increasing gently until the vertical, and progressively relax to weightless near the top.
Doubt it's CoG, you're sitting real close to that point.
Did a loop from S&L in a standard 152 as part of my aero rating; experienced exactly the same thing you described. 'Twas coz I was too eager to try and get it past the vertical before it ran out of oomph and thus had pulled too aggressively on the entry. (Incidentally, at the top of that one, there was virtually no margin between inverted stall buffet and positive stall buffet. The speed was almost zero. Very small elevator movement required.)