1999 xl 1200 ltd blown rear cylinder

[kylechandler1300]

Hi all, I was given a 1999 xl 1200 ltd for $500 with a blown rear cylinder. I did a test on it, it has 110 psi in front two cylinders, and 30 psi in rear.
1.) Is this particular model known for blowing the rear cylinder? If so, what is the usual cause for this?
2.) When I pulled the plug out of the rear (blown 30 psi) cylinder, the color of the plug was black. The front two are a nice brown. This just seems odd to me because too me it sounds like the rear was running rich, and if it was rich then why did it blow?
Thanks, Kyle

[kylechandler1300]

Ok, well I pulled the head, and the front two cylinders are perfectly fine and I won't replace them. They had oil in there and were fine. The rear cylinder was bone dry. No oil on cylinder. The cylinder is either scored or has metal residue from the piston. Im going to try and clean it up with muriatic acid. Since there was no oil being injected to the rear cylinder, are the bearings on the lower part of the rod fine? or will I need to replace the rod with new bearings?
Thanks