I think I remember a thread talking about the small holes around the edge of the petrol filler needing to be aligned correctly for the blowback issue. One of the holes had to be at 12.00 o/c top dead centre.
Yes, I use this system - the internal metal flap's hinge needs to be at 6 o'clock. But it's still a slog to fill. I've often wondered if using a funnel might help. Or having another filler cap - so having two filler caps. If you opened both when filling one, whether that m ight make it easier. An expensive solution.
