with holley carbs, you may need to set the accelerator pump cam (and maybe
the spring - what size/type holley is it?) - may be getting to big of a shot
on the first tap... or to little, but i suspect it is to big - you can
adjust the cam or buy other cams  - and yes, ignition timing can be a cause
also - and with a holley and a possible timing issue - it can be a little
work to tweak it out... but you will be very happy when done!

when the cam was installed, was it installed straight up or with some
advance?

~NortH~

Roy,

It sounds like an accelerator pump adjustment problem to me, moreso than
timing.  I have a '67 Camaro, 5.7L, in similar trim.

I know if the accel pump isn't coming in quick enough, throttle tip in
causes a miss/choke type thing (lean condition).  If the pump is adjusted
too much, you'll get a similar misfire accompanied by a noticeable puff of
black smoke.

Memory is telling me that the adjustment is to have .015" clearance between
the pump actuating lever and the throttle linkage when the throttle is wide
open.

I found the "sweet spot" on mine by starting at Holley's reccomendation and
then moving around a little till the engine ran its best.

Another relatively minor issue that can cause the problem you're describing
is mis adjusted air-idle screws.  Mis-adjustment there would cause other
nasty symptoms as well, not limited to black smoke at idle, and fouled spark
plugs.

I don't really see timing affecting throttle response like you think .  If
the car is idling smooth and doesn't ping, or detontate  under load then
your timing is pretty close to optimal.

I run 12 deg BTDC initial timing on my 350.  Some people run 8, some run as
much as 16.  Every combo is different.

Hope this helps,

Roy