I would agree....go through the carb and make sure everything is free and clear...does it look like anything has been modified...maybe it's lean. My buddy had a problem with his blaster....one of the "petals" broke off of the reeds and somehow magically bent the needle? I'm not claiming that's exactly what happened, but we never found that reed petal...we replaced the reeds and straightened the needle and it still wouldn't run right until we dropped the needle(I'm assuming because it was bent). Just two weekends ago he added Boyeson reeds and re-jetted with new needle and boy what a monster it is on the top end....it never pulled well in 4-6gears but it sure does now.
Good luck,
Don
In the works- Honda/Suzuki mutt with XR600R motivation
Last trike- ATC200x w/"S" motor swap, fueled by nitromethane/methanol and stupidity. I wouldn't recommend it.