This plug is FUGLY!

[BIGSHCLUNK]

Thinking about it I had 2 small blocks way back when that had the issue... they were both valve seals. One I had I did twice after about 30k miles. Like I said water down the carb, that'll clean that up real quick

[JS29]

A leak down test would be in order.  piston up, compressor air thru a fitting in the spark plug hole. listen for air escaping. rings will leak and you can hear it through the PCV hole. valves ether through the exhaust or intake. they make a tool that will allow the valve spring to be removed to check and or replace the valve spring. you will need the compressed air set up to keep the valves from dropping to do this.   

[HP_Cuda]

You guys crack me up.

You always do the easiest to hardest to determine a problem.

First you would do a compression check and then a leak down check. If you are really anal you either pull the heads and replace the seals or you try the compressed air trick holding the valves up and do it.

[71-440]

Italian tuneup. From Wikipedia, the free encyclopedia. An Italian tuneup usually refers to a process whereby a motor vehicle engine is run at full load for extended periods in order to burn carbon buildup from the combustion chambers, spark plugs, and exhaust system

Well crap... My car gets an Italian tune up every time I drive her. 

[blown motor]

Here's the rest of the plugs.

[70 Challenger Lover]

So how many miles on this engine? Some have said oil from bad guides or seals but that means high mileage. I threw out rings didn't seat if it's a fairly recent build.

[Chryco Psycho]

 Looks like 1 cylinder might be running ok
It could be anything , from bad guides & or seals to broken rings or installed poorly upside down or ll the gaps lined up

[70 Challenger Lover]

Agreed. I figured knowing more about the age of the engine would help to identify the issue.

[Chryco Psycho]

The engines is coming apart either way , just need to run some tests to know where to look

[blown motor]

The engine was rebuilt about 8 years ago so I would guess maybe 16,000 miles or less. Maybe @ebodycopper  will chime in here.

[70 Challenger Lover]

You'll have to let us know what you find inside. I like these kind of riddles.

[Strawdawg]

As others have said, it looks like the valve stem seals and or valve guides.  Either way, the heads need to come off because I would expect the back of the valve faces would be caked with the same goo badly

[70 Challenger Lover]

Hard to imagine valve stem seals or guides going bad after only 16000 miles. I wonder if this is a recent problem or if it has always fouled plugs like this.

[73_Cuda_4_Me]

Errr....Have the plug wires been checked/replaced lately?

I've had shorting wires that bleed most of the spark off, or crossfire to another cylinder causing lots of carbon buildup firing at wrong time...

[blown motor]

He talked to the guy who built it and he said he lost the cam. They're pulling the heads today I think.