[SuperIrish]

I've been fiddling round with a minor problem on the LTD that comes up. I at first thought it was a timing issue but turns out it may not be.
When I was tuning the carb with the O2 sensor I could get it to run at .62 at cruise which is very good for a carb'd car, and was going to check the WOT mixture so I floored and checked the meter. Upon declerating and going back to cruise I found the meter reading .85 at cruise. This is something that has occurred a few times, I'd change timing and it would run good, floor it and then it reverts back to not running quite as good. I was sure it was timing, then perhaps a vacuum leak but now think there is something wrong with the carb. Yesterday I took the carb apart and blew out the circuits put it back together. Fired right up ran perfect, no hesitation, no light stumbling at light throttle. so I floored it and after that it was back to the old way again, light stumble after shifts at part throttle, not real smooth at cruise, almost imperceptible lean miss, and the plugs will be a dark tan. I've set the timing at 6BTDC and plugged vacuum advance line with same results. Timing seems to have no effect on this it appears to be in the carb. The secondaries are closing fully, there is no fuel dripping into any of the carb throats, and the flaots are set right.
It's an '88 HO shortblock, ported DOOE heads, Performer RPM intake, Kaufmann Stage III roller cam, '85 GT distributor, Edelbrock 750 carb, 10BTDC timing, new distributior cap and rotor, wires and plugs are about 6 mos old. If you've got any ideas, suggestions or want more info let me know.