Update - Progress!
 

1. Lots of carbon/soot on the plugs (checked 2).
2. Idle transfer slot, as found: About .060 uncovered primary, completely covered on the secondary side.
As left: About .030 primary, completely covered secondary (no change made)
3. Idle mixture screws: As left: About 1 turn out, primary; about 1/2 turn out secondary.
Results were very encouraging but not 100%. Off-idle hesitation is much less severe, but still present. During the mixture screw adjustments, at no time did manifold vacuum improve when I (gently) tapped the accel pump lever. Adding fuel was always a detrimental effect. So it wants more air. I'm not finished tinkering with the idle mixture just yet (I'm going to try the technique advocated by Don Gould at www.4secondsflat.com, which is very similar to the advice given here), but I 'm thinking about a small hole in the primary butterflies.
Thanks for all the suggestions and guidance, and please keep it coming.
CWizard

Get a very small portion, about .020 of the secondary transition slots exposed, it should look like a very small square. You will then find it necesssary to readjust idle mixture screws. Adjust mixture screws for highest idle speed. Ideally they will be about 3/4 of a turn open (all 4). , then adjust throttle adjusting screw to reduce/increase idle speed as necessary.

Good Luck

Sounds great! Nobody seems to beleive when you tell them about the initial timing and the transfer slot relationships to the whole idle and off idle performance. No you know! Don't worry to much about the vacuum, give it what it wants. The holes in the butterflys are only required when you can not get the whole timing, transfer slot, idle mixture screw thing right and the idle speed is to low.

Wizard....your carb issues sound exactly what I went through with my 418W last week. I pulled the plugs to change them and they were very sooty. This is because I was idling too much off the transfer circuit and not the idle circuit. I initially had both the primary and secondary throttle blades opened for a balanced idle on all 4 corners. What I did next was reduce the idle speed by turning down the idle speed screw on the secondaries so it was barely open. I turned my idle speed on the primaries down also to an idle speed of 900 rpm. This seems to be the lowest idle rpm that my motor is happy at. Next I turned in the mixtures screws, increasing the idle speed. Then I again turned the primary idle speed screw back down to return engine speed to 900 rpm. What this did was allow me to adjust the throttle blades away from the transfer slot and closer to the idle port. I expect the idle mixture will be cleaner at this point. If you turn in your mixtures screws and there is no change in idle speed, guaranteed you are working off the very rich transition circuit and not the idle circuit. This makes for a very dirty idle mixture. As said before, if this fails you may need to drill some small holes in the throttle blades to allow more air into the motor withouit opening the throttle blades into the transfer slots. Just start at drilling a 1/16" hole first. I'm sure your motor is just as fussy as mine trying to get this idle mixture and off idle stumble as clean as possible.

Bill