Now I have a new plug plus the muffler has a gasket. It seems slightly better but still vibrates at idle.

I ran the engine today and flew the yak a couple of flights. Just a brief hover and some knife edges and couple of loops. I noticed when inverted there is, but am not 100% sure, small hesitation in the engine. Would a rich setting cause hesitation while inverted maybe?

Also, the engine has just been broken in, and had only few flights (5-6 flights) with synthetic oil (redline 1:40 mix). And, there has not been any changes to the factory low/high needle settings.