Help...Engine backfires!!
 

I'm getting an occasional backfire through the carb. The first time it happened I had filled up at a marina I'd never gone to before and it did it about 30 minutes after. Ok...I'm thinking water in the gas. Dry gas added to tank. Still did it, so I replace the cap, rotor and plugs. Plugs looked good brown to black in color, none I would call fouled. Rotor had rust in center. Still happening. It sound like a cough or a sneeze. It happens while on plane going a constant speed. I have black stuff on the under side of the engine hatch around the air filter from the backfire. I also get some black around the exauhst but thats always been present. I don't think its lean. New this year are Holley blue electric fuel pump, water seperater / filter, pressure regulator (showing 5 lbs on the gage) . It seems to happen when:

1) I'm on plane at a constant speed
2) I've been running at least 15-20 min
3) It happens about once every 20 min.

I'm very confused please help!!! I'm worried what its doing to the motor.

What I've got:

1988 Formula 242ls
502 GM crate motor
Merlin heads
Dart Intake
Msd Ignition, rev limiter, distributer
Taylor wires
Blaster Ignition coil
Holley carb (unknown model, how do I tell?)

Here are some pics:

Carb

How do I clean this? I have not change this filter. Looks to be made from brass.

Are these wires bent to much? Should I try new wires?

Coil. Coud this be going out occasionaly?

Any one know what the white hard plastic tubing is. Looks like it should go near the distributer somewhere.

Underside of engine hatch, no more pics. Please help!!!

How about an intake valve sticking after things get warm?

Is your oil fresh?

Oil is due for a change.