Just passing by and want to give you some extra info on the R2105

OS is using a different crankcase on this engine which has a lower volume in porting and explains the improvement in rpm climbing.
With this they also use a new main-needle with 2 orings and this makes the engine less sensitive on the main-needle.
Also a different spraybar is used(thinner shape) not sure what this should bring but a change is there too.