Put the extra shim back in. You are having to use a colder plug right now to help bring the timing back to where it needs t be, but instead put the extra shim and and continue using P6 and P3 plugs and you should be fine.

Galifrey- Just because your engine runs fine on 30% and two shims doesn't mean that everyones will. As I have said before there are more variables than anything in the tuning of these tiny little engines. Everything from fuel brand, makeup, barometric pressure, ambient temperature, elevation, humidity, pipe, fuel tubing, gearing etc. etc. etc. can effect these engine. And judging from your last post you obviously don't understand this.
Yes he could have an air leak or a bad carb- but the shim is the first thing I would try before searching for an air leak or bad bearing.