Here's another data-point.

We fitted a Thunder Tiger 46Pro muffler to one of the GMS 47s in our club and it significantly improved the reliability.

Power was down but the damned thing didn't miss a beat no matter how much we tried to make it die.

Perhaps the extra restriction of the TT muffler is creating more muffler-pressure which would support my theory that these engines are over-carb'd.

I've also noticed that the GMS mufflers have their pressure tap right smack in the middle, not necessarily the best place -- ideally it should be in the place of maximum stagnation because the slower the gases are moving the greater their pressure. I'm going to try moving the pressure tap on the GMS muffler and see what happens.