fuel mixture help needed

hi got a 1975 st70 and need to no what mixture if any it needs i have put unleaded in but will only run on choke very lumpy last time i had a st70 4 star was what i run it on ! thanks

You can run on unleaded fuel without any problem.
Most likely:
1. dirt in the tank / flue lines / carb
2. carb settings (if you changed them recently)
3. air leaks