Please help with engine trouble

[darush01]

My 72 c20 is my dd and the other day I burned up a plug wire on the exhaust and once I replaced it it ran a little better but still bad, I did some checking replaced another plug wire bc when I pulled the wire from the cap with the engine running nothing happened then I replaced the plug because it had some white substance built up and couldn't have been firing, but it still didn't fix the problem I plan on replacing the whole set of plugs and wires ASAP but I don't think that's the whole issue, it's got a newer set of points and a newer cap and rotor, I'm lost I think at least that cylinder is not doing anything and it was kinda sudden I mean one minute it's fine the next it's not please any ideas?