It could be that the carb needs to be removed, torn down, soaked in solution and rebuilt. If there has been even the slightest dirt or varnish in the carb, that could cause it to run poorly. Todays gasoline seems very poor to start with. Since it is so new, the jets probably are not adjustable. I would doubt it was in the ignition also since it is so new. We have seen some of these newer carbs do the same thing, and normally they just replace the carb because they can't be adjusted. But, I'd try soaking the carb in solvent, after removing the bowl, float, and any gaskets/orings etc. If that doesn't do the trick, you could check for strong spark. There is a tester that goes inbetween the spark plug and the wire that shows the spark in a tube, and that would tell you if you had an ignition problem. Again, sounds like a fuel problem to me though.
Is there any chance there is water in the gas? Have you taken the bowl off the bottom of the carb and looked at that?
