The 12, 13, 17 means A/F ratio. I suspect Versy is using an A/F meter.
Any slide type carburetor (CV or VV) can have a problem where WOT fuel is limited by the Needle Jet (NJ) instead of the Main Jet (MJ). It's unusual, but I have personally experienced it and also read about it. You can eliminate the NJ as a possible source of the lean-out by simply replacing the main jet with a grossly oversize jet. A main jet so large that it should be virtually impossible for the mixture NOT to go rich. If the engine still goes lean at WOT, you may need a larger needle jet, or possibly a smaller needle, or a compound needle.
I read one article where the author actually removed the main jet entirely to facilitate the test. That seems pretty stupid to me since the needle jet could come out and go through the engine. I bet that would be ugly.
You also might have an issue with fuel delivery. The needle and seat might be too small to allow sufficient flow to support WOT, but large enough to support part throttle operation. It could also be an issue with restricted delivery through the petcock or fuel line.
Sounds like you've got one thirsty motor. Those are generally a lot of fun