I had the same issue but it only works when I take the cover completely off 🤔

 

I assume by that you mean the bike runs better without the airbox lid. If so, the original poster's ideas of cleaning the carb out, checking it has the right jets and possibly altering the main jet needle height, should fix it. Plug colour with the box lid on should tell you if it is weak or rich. Although intuition would indicate taking air box top off makes the mixture weaker, this is not always the case. (I believe the 250 Honda runs richer without lid). If it is running rich with lid on, check the choke is working properly, though this gives more trouble at low revs than wide open.