Btw, you didn't say if hte engine was new or well broken in prior to being mounted inverted. As you may know, a good, low, stable idle takes a while to develop, espeically on the larger ringed engines. So that might be part of your problem as well.

flycatch, makes a couple of good points that can be generalised to any 4-cycle plugs, or the Fox Miracle plug will help, they are hotter than most 2 stroke plugs. You already said you went to a hotter plug, maybe going hotter still will help.

More nitro helps most engines idle better and makes them less sensitive to mixture settings. Since half the problem with an inverted engine is getting the idle mixture sorted out, more nitro from any brand fuel is going to be a good thing.

2k for an inverted engine sounds pretty darn good to me.