<VV> Carb suggestions?

[Grant Young]

Looking for some input on what is potentially a carburation problem on a 1953 Corvette in-line six with the 3 Carter YH carbs. I rebuilt the carbs for the owner several months ago. What he sent me were some hybrids that had parts from Corvair and Nash carbs that had been swapped into the Corvette units over the years. I put them all back to correct matched specifications. The mixture is now apparently too rich toward the front of the engine and improves toward the rear. The Corvette YH carbs have manual chokes (which were syncronized when they left me) and smaller main jets (.080"). The Corvette carb is much more sensitive to throttle plate position at idle, so I think that might be it, but here is a copy of the note I got from him today: (A private reply is fine if there is no Corvair knowledge to be gleaned from this. Thanks in advance for any advice).

"The car runs much better than ever. The front 2 spark plugs (1 & 2 ) are still black and sooty. The middle 2 (3&4) are light brown. The rear 2 (5&6) are almost white . When accelerating there is a small popping sound coming from 1 tail pipe. If I change the 2 front plugs, it runs great for a while. Can you make any suggestions on what to change or adjust?" 

Thanks,
Grant