Hello ! my two cents worth , NGK B4h are what I use also , the primitave engine won't benefit from the new " cookin dude " plugs ,, .015 plug gap is plenty , large gaps tax coils , If it runs fine there is no need for huge spark clearances ,
also I use opti 2 mix at 50 / 1, engine runs cool and very little smoke , carbon issues have been almost non existant ,, don't make mistake of adding oil to mixture , extra oil will lean out the mixture ,replacing fuel with oil , which in turn mixed with air , will LEAN the muixture making the engine run hotter, (honest engine ), no outboard oil either , flashpoint only suitable for water cooled engine , will become less effictave at air cooled engine tempatures < ,,,break in engine slow over a long time and STOP if any sign of seizure (loss of power ) is noticed , a liottle time on the side of the road will pay off in little or no repairs
happt riding Y'all Larry.