I had this problem with a Webra 120. I checked all the usual suspects to no avail. We spent hours checking fuel line, o-rings, tank, blah, blah, blah. Guess what, it was number 2. Low speed mixture set too lean. It was so lean that the spray bar was still mostly blocked by the low speed needle at w.o.t. One of the teenage kids at my field suggested checking it. The old timers learned a thing that day.