I'm flying a Goldberg Skylane 62. Semi-semetrical high wing, small amount of dihedral. When I do a roll, the roll rate nearly doubles when the plane is inverted as soon as I add down elevator to maintain altitude. I'm fairly certain that it's the application of down that causes the rate to go up. I'm not using any rudder during the roll because the coupling effect is too strong. Anybody know why this happens?

Pendulum effect? Since it is a high wing, it has more inertia coming out of inverted.

just a stability thing
most NON aerobatic stuf is (if any thought went into it) setup to be stable , not nuetral or as-stable or neutral stable etc.,

I always figured it was because you are fighting the natural tendancy of the dihedral to keep your plane right side up during the first half of the roll, and then the dihedral is helping the roll during the second half.

It is...partly. The dihedral in a high-wing plane creates stability through aerodynamic effects and the pendulum effects. So, yes you're right....indirectly.

When you're upside down with a Skylane the airfoi now had a negative camber value. With such an airfoil the likelyhood of the airfoil stalling and inducing a snap roll condition is quite likely. Not to mention that you just turned a fairly extreme highwing design into a fairly extreme low wing design. Add all these together and we have potatoe salad.

Your Skylane shows this trait but it's due to a fairly unique set of charactaristics. Try flying a funfly or 3D model and it can't tell the difference from wheels up to wheels down and responds exactly the same either way.

The snaproll makes sense, since it really happens when I give it the up elevator. I suppose it's time to convert this one to a float plane and get myself an aerobat. Thanks for the answers.

It is indeed just a roll stability effect as was noted. The Skylalne 62 being a high wing airplane has a higher roll stability upright than when inverted.

If you give it a roll control (keep it constant throughout the roll) the airplane starts to roll against this stability, accelerating until a maximum roll rate is achieved upright. This occurs probably during the first quarter of the roll. As the roll goes past the first 90 degrees the roll stability is much less and allows the roll to accelerate more and the roll rate increases. That higher rate will be there between the 180 and 270 degree roll angle, until the airplane goes back toward the upright flight, when the increased toll stability will tend to slow the roll down some. If you hold the aileron in you'll find the roll rate will oscillate.

If you have a high wing airplane with a whole lot of dihedral you could see the effect even more.

Ben