Hi!
The .40-,50 Webra is not any differnt than any other two and twostroke engine ! they could swing any prop, large and small, without problem...just set your needle accordingly!
I´have run the webra .40 GT in my Q-500 pylonracers for many years and most pyloncompetitors in sweden use this engine as it is by far the most powerfull engine of all .40 twostrokes on the market (we use only 10x6 props and FAI fuel in Q-500 pylon).
The Webra trottles exeptionaly well and rews at 14000 on a Graupner 10x6 prop, a lfew hundred rpm less with a APC.
The Webra .40 is equiped with a Rossi 3 and I think the .50 Webra also use this plug.
It might be that you run to much nitro that you have a problem...these engines are made to run on no or low nitro blends as nitro in central Europe are hard to obtain. Here in Sweden it is no problem to get it as dragracing is very popular....but it costs 13$ for 1 liter though.!!

Regards!
Jan K
Sweden