<VV> Occasional total loss of accelerator response at stoplights

[chuck mckinley]

It's been about four months since I did a major carb cleaning & rebuild
on my '63 Monza, and in recent weeks I've begun to experience a behavior
that's bound to be carb-related. Very rarely (like once a week) I'll be
stopped at a traffic light, and when the light changes, I get no
throttle response whatsoever until I pump the accelerator a few times.
My last carb rebuild was with rebuild kits from FLAPS (not Clark's), and
I'm sure they used the old-style pump diaphragms. I've heard tales (on
this forum) about how they're subject to "ethanol rot", and I suppose
that's a distinct possibility, but do those symptoms suggest any other
likely causes for this behavior? When I say no throttle response, I mean
it quite literally - it's like the accelerator isn't even linked to the
carbs. And it only happens when I'm stopped and have been idling for a
minute or two. I'm ready to tear into the carbs again, but would like
some hints about where to focus, if my description provides any useful
clues.
Thanks as always (in advance) for your expertise.

Cheers,
Chuck McKinley
'63 Monza 900 80 hp 3-speed
'79 Honda CX500 (new toy!)