Your jetting should be 290 main and 27.5 pilot, middle clip on the needle, and 1.5 turns out on the pilot air screws... Don't use a BR9ES plug it is one step COLDER than stock... NGK heat ranges run backwards, i.e a 9 is colder than an 8, and a 7 is hotter than an 8... I had the same problem with my stock motor, I rebuilt the top end and it was fine... It ended up having an egg shaped bore, causing it not to seal correctly when it was cold, it would slightly clean up after it got hot. But, was not perfect until after a bore job and new pistons and ring sets..