| Make sure your using a good quality sparkplug NGK is the usual choice
but worry about tweaking it after 1-2 tanks of fuel it may just be tight in the run in period so keep the fuel around 32:1 until its had at least 2 tanks through it then switch to the 40:1 ratio
so for now try a new plug & run the engine in at least 2 tanks of fuel first, it may just be rich down low & clear up after run in period | 
