There aren't many places for it to bind. With the rotor head off you said the rpms are symmetrical--this is hard to determine on an unloaded motor without a tach but I'll take your word for it.
Do you have any end play at all in your main shaft? There should be none but it should not be soo tight that the bearing is loaded in thrust--in technical terms the end play should be 0 to +0.0005, that is not squeezing tight but no more than half-a thou play. This is of course adjusted by the shaft retaining collar, which by the way must be on the right way up (coned part toward the bearing). And of course your bearings are properly seated in the fuselage mounts.
With the end play set correctly, and with the head off, if you push up with your finger on the bottom of the main gear does it slow down substantially? If so then you need new bearings. They are only like seven dollars for the pair, I'd replace them just on spec.