Backfiring thru the carbs??

'93 SS with the stock carbs. Just finished replacing the "emulsion tubes", and I'm enjoying much better gas mileage, but there is an annoying backfire that happens when the bike is cold and the choke is off. I get a block or two from home, stop at an intersection and "cough" the engine dies. Starts right up without the choke seems to run fine for a while except for the random backfire. Seems to happen less often when she is warmed up. Any ideas as to where I should look for the source of this problem?
Thanks,
David