Hey Mudge, just went through 5 pages of your ordeal, I hope I might be of service to you or whoever might be reading this in the future.
First some DGV Z car tips:
1.Make sure ignition system is in top shape.
2.Rebuild the carbs. Kits can be had in the $20 range (each) and
include the power valve and accelerator pump diaphragms, which
frequently go bad. It's very easy to rebuild these carbs if you have
at least average mechanical skills. It also gives you the chance to
clean the jet orfices and float bowl.
3.Connect the vacuum advance hose from your distributor to
the vacuum fitting on one of your carbs, NOT to the intake
itself. If you connect it to the intake, the vacuum advance will
be wide open all the time. Plug the vacuum fitting on the other
carb.
4.Go through your linkage to make sure everything is synched up
and not binding. It's not a bad idea to disconnect the linkage
when adjusting the idle speed and mixture.
5.Adjust the the idle speed screws as low as possible ( till the
engine rpm gets as low as car can take and still idle reliably).
6.Have the idle MIXTURE screws out about 2-3 turns. Start on one
carb and turn the screw in at about 1/4 turn increments, until you
hear the engine start to stumble. Back the screw out until the engine
starts to regain smooth idle. It's been my experience here to use a
vacuum gauge connected to the INTAKE, and to back the screw out
till the gauge reaches it's maximum reading, then stop. You MIGHT
turn the screw in a hair at this point.
7.Repeat step 6 for the other carb.
8.Adjust your idle SPEED screws to your liking ( I prefer about 800
to 850 rpm). Adjustment back or forward on one of the screws may
be necessary until you get a good even idle.
9.If you're still experiencing an off idle hesitation/stumble, adjusting
your timing up may help. Common Z car practice is to experiment
on pushing it up until you're pinging at top gear under load, then
retard it back a bit.
10.Jets? In my opinion, the most important one is the primary idle jet.
I have a stock 260 with these carbs, and even after all these above
adjustments, I still had a bit of stumble. I was running a 50 in the
primary idle jet, a 55 in the secondary. I did what you did and
switched them, 55 primary idle, 50 secondary. Definite improvement
I just did this, so time will tell if I got rid of the stumble for good.
I'll post again in a couple of days with the results.
Hope this helps someone.