I am running pitts style mufflers on 2 of my 3 older Webra 120s and of course that effects how they run. Both turn a 16/6 apc in the high 9s. I've never run a GMS 120 but I've picked them up and fondled them at MECOA in Monrovia. Definitely look like a Webra copy but I didn't look at the carb that close.

I normally adjust my idle for best good transition to full power after a few seconds of idle. This normally leaves them a bit rich. But that's OK as long as they don't load up in the mid range. This doesn't seem to be lean enough for my Webras. What I do is lean them some more and then start flying. If they still load in the mid range I land and turn them in a half screw driver width at a time till they clean up. This seems to work for me.

Denis