running it into the upper rpm's 'slow & easy' through the low gears should find a fuel supply issue, that's where mine displayed chugging as the stock mechanical pump couldn't supply too many carbs. turn smooth rpm over a long period of time to use as much fuel as possible
did the carb dry out during the rebuild process? it may just need a 'going through', could be dried crispies floating around in the bowl. I've even seen mystery extra parts inside a carb on factory new bought vehicle. every now & again they'd get hung up on something in the bowl & the vehicle was crippled. limp it along for a while & it would fix itself. removed the extra parts & it never happened again. the parts didn't even appear on the exploded view diagram