muffler blew up, mechanic said running rich

Bought my 90 LS about 4 weeks ago w/ 70,000. Muffler was fine, but then started making noises (exhaust leak) and then i noticed that the muffler was falling apart. So i went to meineke to get a regular muffler put on, because of their lifetime warranty. The guy there said the muffler blew because the engine was running rich and this type of malfunction is not covered under their warranty. He said i should look into getting the timing done… How do i do this and is it expensive? Will this fix the problem? Anything else should ii lookk into… thanks for the help all;)