[Jac Mac]

I wonder if it really was the top filter or other factors:
1. Could the filter top be too close to the carb vent tubes.
2. Is air really entering the engine bay/carb via the scoop . Do a (wool tuft) test while driving & check what its doin out there.
3. Lots of hot air gathers at highest point under the hood, if there is not a lot of air going in the scoop, guess what your engine is now trying to breath.
4. With that in mind air cleaner should really be sealed to the scoop.