First ck your spark color, blue is what you need, second go to a hotter
plug . Did you ck compression, what you describe is not the gas but just
an older motor , rich running, weak internals burning oil, or weak
spark. Keeping the same heat range will do nothing even with a different
type plug. Also a plug should fire through mild carbon , a dark color
plug is normal in small motor design . raise plug temperature and ck
everything else, but not firing may be just weak spark. You dont want a
yellow spark, thats a weak spark.