Changed my spark plugs yesterday and noticed the back cylinders were quite black but the fronts seem to run quite clean. It's a Holley 4160 on an Edelbrock Performer. What would cause this?

Thanks

Black from excess fuel; or black from oil?

If black from fuel, is the engine tilted rearwards more than usual?

If black from oil, is it rings, guides, or PCV?

Hmm... I'd say fuel. Would be surprised if that pattern was caused by oil as well as it's quite even on both sides. Car is very stock setup and the engine is not tilted that I can see.

Thanks