loads up like in carboned plugs?
low compression will mimic rich mixture.
as will retarded timing to some extent.
make sure all the jets and orifaces are clear...and jets are tight.
a float not floating at the right level can act rich. if they're brass check for pin holes. shake them and listen for sloosh
crap under a float valve can also act rich.
did the problem begin with the carb rebuild?
lastly I'm assuming the air filter passes air.
...and I'm assuming when you say "loads up" you are talking about a rich mixture???