Are larger Jets even available for it? If you could find out the diameter of say a Mikuni 300 and find a nibbie of equal size that might give you a starting point. ALWAYS want to start large and work your way down. Buy a box of plugs and read up on how to plug chop. You need to give more info. Air box and type of filter, mods to inside of engine, temperature, and elevation.
What size is the carb
Normally you’d be 270-280 with stock carb. But because we’ve never seen success with those you need to start real rich .
IF you get it jetted without melting a piston PLEASE post your results