Peter,
Surely the TS250 4speed runs on 50:1 petroil, as do the Supa5 and the early ETZ250 Standard model, even thought they all have different size main jets. I think the ETZ Standard and Luxus models both had a 130 jet even though one used prexix and the other did not. If this is correct, the changes in jet size were unrelated to the use of premix or its ratio.