Could be a bad plug. I had a 1.40L that drove me nuts and it turned out I replaced a bad plug with a another bad plug. I actually sent it to Richard for repair and he called me and asked where I got the plug that as in it. I told him it was in my field box and he said it wasn't a YS or OS plug. It was another brand from when I was playing with other four strokes years ago and my YS did not like it. He did nothing more than put a new YS #4 in it and send it back. Ran great until I sold the plane it was in. I still have that engine and it still has the plug Richard put in it.

Run down the list of usual suspect above....unless you have a servo oscillating at idle or a REALLY soft mount allowing the engine to move a lot what is listed usually clears things up.