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There is nothing unique about a 20x6 prop that will cause your problem, other than the fact that your engine will rev higher and may require more fuel. Normally, with the needles peaked, this would not be true, but with the needles set to run rich, it is a possibility.

Engines will surge when the fuel tank venting system is compromised or the fuel pickup clunk is too close to the back of the tank. Another possibility is that the brass vent tube in the fuel tank is too close to the top of the tank, blocking off air from entering and then fuel from exiting the tank.

I agree with Silversurfer on the break-in issue. All you are doing by running a gas engine rich is prematurely coking up the interior of the engine. This practice should be left for glow engines, where it does make a contribution to the engine's break-in by helping remove excess heat via the expelled unburned glow fuel. Gasoline does not provide the cooling advantage of alcohol. However, on the plus side, gasoline is a lubricant of sorts, whereas alcohol in glow fuel is not.