and you are sure the cam is degreed correctly?
have you checked the spark to make sure it isn't breaking up at low rpm?
and have you pulled the bowls off the carb yet to make sure everything is OK in there?
B/c one time i bought a carb of ebay, looked brand new (advert said it was new, installed and removed b/c it was too big hehe).
bolted it on and it fired right up but wouldn't run, rolled black smoke and died. I pulled the bowl and there were no jets. put jets in and it ran fine.
as notedthe jets could be gone, the power valve could be torn or even installed without a gasket...