Over the last couple of weeks I have noticed if my truck sits for 2 or more days I get A LOT of black smoke when I accelerate out on the highway. It happens for a bout 20-30 seconds and doesn't happen again. I drive about 1/2 mile from my driveway to the highway at low speed. Get on the highway and accelerate up to 60 mph belching black smoke the entire time. It gives a HUGE cloud and then keeps puffing small dark clouds. This is in the stock setting. Where should I start looking and what can I test before throwing parts at it. Seems to me I have some fuel leaking down over a couple nights causing tons of black smoke? Any thoughts?

EDIT:

Only mods I have are a DP Tuner, 6637 and 4" exhaust.

 

Anybody got any thoughts on this?