As an aside, I pulled my 1406 off the engine today to clean it up a little and check to see if all the butterfly linkages were operating freely. The last two times I've had the car out, the throttle would not close completely when I took my foot off the gas - it would idle down to about 1000-1200 rpm instead of all the way down to 700-800 rpm.
Anyway, I noticed while I had it off that the accelerator pump was not shooting any fuel on the very first advance of the throttle. It would start to shoot after about an 1/8" of throttle travel. Also, when I removed the linkage between the throttle and the pump and moved the pump up and down I feel a momentary but noticable 'catch' or hangup as the pump is returning after depression. My diagnosis is that it needs a new accelerator pump for starters. And seeing that the pump spring seems to provide the last bit of closure on the primary butterflys (through the linkage) that may also be part of the problem.
Anyway, the only reason I mention this in this thread is the reported stumble by the OP (as well as my stumble although slightly different) after stabbing the throttle, which I believe is due to the accelerator pump not delivering enough fuel immediately causing a momentary lean condition until the secondarys kick in and remedy the situation. I'll let you know after I replace the pump.