The Enya 40SS has been made in many flavors.... air bleed carb, twin needle carb, bushed, twin bearings, iron piston/steel sleve, and I think some were made with ringed aluminum pistons.

If yours is iron piston / steel sleeve (the most common) then you may need to run it on 20% castor. That is the engine I have and it seems to run best on high castor.