The flash channel on your Lehigh plug should be 5/32. Just use a 1/8th bit to start cutting the primer carbon out spinning by hand. Then move up to the 5/32 bit and do the same. If I am wrong about the 5/32 bit someone will correct me. I have not shot nor cleaned the two of mine in awhile.
BH209 breechplug cleaning:
http://www.blackhorn209.com/specs/breech-plug-cleaning/
Good luck hunting!!