In my MANY years in the hobby, I've never heard anything like this. It sounds like you have a TINY ASS air leak somewhere, that's all I could think of. Swap your tank, lines, and all filters. Seal the backplate, head, and carb base. And try EVERYTHING again. I don't know what to tell you tuning wise, seems like you've tried everything. If all else fails, I have a nice RB engine I can sell you for a good price, if you're frustrated with this engine. It sounds as if you got one of the earlier ones with the carb trouble, as SpeedBump described in an earlier post.
Anyway, PM me if you just want to start out with a new motor, or we can go back to the drawing board to get yours running right.
Anyway, best of luck selling the older K Car and the buggy, I know a guy who may buy up the buggy so keep your eyes peeled on the auction.
Good Luck!!!!!