[Mgs] fouled plugs Why?

[melfrankus at carolina.rr.com]

I need advice.  My 71 BGT has no factory warranty remaining. 

 The engine lost power. Oil presssure reads 40# at idle, engine runs cool, no water in oil, carburators are carbing or whatever they do. 

 Deductive reasoning and process of elimination revealed that the two middle spark plugs were not firing, were blackish and smelled like fuel.  I replaced the two ugly plugs with new spark  plugs.  Other plugs are ok.

The car runs fine now, but I want to know what caused the two spark plugs to go bad.

I am not capable of going beyond simple tune up type work.  Can someone on the list tell me what internal problem might have  caused the two plugs to foul out????

Thanks, Mel