Nailhead help! Popping through exhaust

Engine is a stock 401 Nailhead with new dual quads on a Offy intake, and home made fenderwell headers (open, no exhaust). With the timing advanced, it's a little tough to start, retarded and it fires easy. Either way though, it keeps randomly popping pretty hard out of the left header only. And it only occurs at a coasting idle. Not sitting still or under throttle. Pulled the plugs out on that side and they all look the same, slightly white/brown with a dry/heat smell.
Am I running too lean with the carbs? Timing way off? Holes in the header? Internal mechanical issues? (hopefully not!) ...any help would be great, thanks
-Dean

 

Dean...you say coasting idle....how about on deceleration? I would say that it could be your headers might be allowing air to be ****ed in on coasting/decel which causes unburnt fuel to pop in the pipe like you mention...make sure the side that is popping has a good gasket and that its not leaking...

 

Spark too late and too rich mixture on carbs?
I quess that problem is with ignition timing ("tough to start")

How did you set up your dual quads? Not progressively, not?

 

I'm with black panther .... my guess would be air leak on the header.

 

What carbs are you using?
3X air leak at header gasket. look underneath....

 

Going out to look into it right now, hopefully it is just the header. Carbs are Edelbrock 550cfm dual quad carbs, set up progressive. Thanks,
-Dean

 

id say do a static compression test, running compression test and a leak down test. ive seen uneven compression cause noise like what you describe even in full exhaust piped cars.

 

I think stick with the easy stuff first and check for air leak at the header flange. Just as several suggested.

 

Turned out I had a fuel mixture screw needle on the primary carb snapped off in the carb! And the vacuum advance on the dizzy needed manifold vacuum, not ported. Seems to be running great now, just trying to adjust to a happy medium between too rich and too lean.
-Dean