Probably the regulator assuming all else is well. First double check that all really is well. Specifically check for

Tank holding pressure and no fuel leaking out of engine (engine off)
Gaskets, o-rings in good condition, etc.
Good plug, OS #F or YS (I'd change if you have a spare, even if it looks OK)
Valve clearance is set properly
Engine is propped correctly
Proper fuel is being used, i.e. 20/20 or similar
Needles and regulator seem to be at a reasonable setting (reg should be roughly even with housing)

If all that checks out, and the engine is still loading up with fuel midrange, start adjusting the regulator. If you are going fat, lean the regulator 1/4 turn. Do this by turning clockwise, which compresses the regulator spring so less fuel is let by. Note you may have to reset your other needles after making a regulator change. Be sure to test with a warm engine, but don't run extended on the ground or it can overheat and then you end up chasing mixture all over the place. If you still need more adjustment, keep turning the regulator 1/4 turn. Fine tune as required. However, if you need to go more than 1 full turn from flush of the housing, something else is wrong IMO.

FYI - every YS I own I've needed to adjust the regulator some, but typically never more than 1/2 turn.

Cheers.