Why "dieseling" after ignition off?

Yes, higher octane may stop it, try it.

If you’re not already for the time being, until you work it out, engage first gear and slip the clutch to mechanically force the engine to stop. I used to do this on my Triumph.

Set idle very low after ensuring all carbs are synced

Then pull choke just before you turn off ignition
Rich mixture cools combustion chamber