RE: XYZ 53cc Twin
I richened mine up just a tiny bit the other day and it made a BIG difference. I think the carb settings are just touchy. It almost died around half throttle - as in - it skips a few beats but then kept going. I landed and leaned it back out, ran like a champ. I'm not sure what to tell you on your motor but my guess is that you're too rich.
Here's two videos of my XZY running.
[youtube]http://www.youtube.com/watch?v=P9clw0A_9iM[/youtube]
[youtube]http://www.youtube.com/watch?v=75SVkLMOxXU[/youtube]