Another possibility is that something is blocking your main jet or the passageway to it. Especially with a 1 barrel, if any trash/dirt/piece of old gasket, etc. blocks or partially blocks it will make the motor go lean.

You really have to make sure all the passages in the carb are clean. Today's gas is really bad about gelling if it sits for very long, so when the carb is apart you have to make sure you clean everything using compressed air to blow out all the little ports to make sure it's absolutely clean inside.