Be aware that (assuming no other problems) the most likely reason for rich running at idle on one of these is wear on the top/thickest part of the metering needle and of the jet orifice that the needle goes into (IYSWIM)

You can compensate for this by adjusting the jet position but the likely outcome is weak mixture and loss of power at higher speed/loads.

You may need to consider the possibility of replacing the needle/jet if this proves to be the case.