I have also had this same symptom with some new 3W engines. Although the other suggestions are all good ones for most any gas engine, I have found that this issue goes away completely with time on the engine and the leaning out that comes with it. After you get past break in and are on full synthetic at 50-1, and the needles are tweaked, all these problems seem to go away on mine. I had the same problem with the 3W 55US that I just got broke in last year. I also cut those looong tillison needles down to size (like Walbro) and re-slotted them, while I was at it.

I know it isn''t fun to go through the process, but my engines never quit altogether, just an annoying sag, till the came in. It is worth the wait for these power houses. Better have your 5-way on.