My understanding of that adjustment screw on a vacuum carb was simply to keep the throttle plates from getting "stuck" in their respective throttle passages. Of course with it slightly cracked you are pulling a touch of fuel which will help the idle circuit. On a double pumper, it definitely helps to use it as it allows you to tune the primary idle screw much better.
So do you think the 1/32 to 1/64" crack open is fine? I watched with a bright flashlight and couldn't see any fuel. And now that I think of it, with the linkage off, the butterflies did indeed stick a little bit.
I used to run the Holley 3310-1 quite a lot on different engines - mainly because it was easy to adapt to them. The carb was a 780 CFM with dual metering blocks each with a power valve. I always set the secondary throttle plates to just barely bump open with that screw. I ran that carb on small blocks with decent cams and 350/350 stuff plus a few big blocks - a 454 being the biggest. Nothing super radical. I would think the carb you have right now would be very similar. I also just went back and read some more of your posts - you might want to unscrew the base from the carb body, verify with a metal straight edge top and bottom that it's not out of true and also check the bottom of the carb body. Unless something is out of whack, those secondaries should close cleanly by themselves - and I never really thought about the vacuum spring providing any assistance. I will pull one of my old vacuum bases out and check but I am pretty sure if I open the primary all the way and the secondary by hand, that it should all close when I release the primary throttle.
chas, I bent the linkage but I also ordered a new one. We'll see if it's correct. The spring doesn't do much at all but slow the opening, it's up to the primary to close the secondry. I did find the spring does do a lot more than I thought in slowing it down, but still with the weakest spring it's pretty slow.
Before I bent the linkage I put in a stronger spring and it really made it open slow even with my modification. I remove the check valve ball (which is made to leak slow) and with a phillips screwdriver stake the seat making it leak faster. It's tricky, if you go too far you have to put the ball back in and give it a few wacks so it seals a bit better. I had told a guy at Holley about that long ago and he said he does the same thing, so did a guy at Summit. I thought I was the first to think of that, oh well.
Secondary throttle stop: Adjust so the throttle plates do not stick in the bores.
IF you need to adjust the idle speed screw to far to maintain desired idle rpm & the carb transitions into the main metering system @ idle then adjust the secondary throttle stop a bit to allow more air into the engine & get the primary back on the idle circuit.
55 Tony - I just checked an old 650 base plate and we're both correct. It closes as you let the primary close.
There's an adjustable secondary vacuum kit that you might be interested in. QuickFuel appears to have the best price at $30 but you can also make your own. A small hardware knob with a short piece of threaded rod and matching nut will do the trick. You will also need a tap that matches the threaded rod and drill bit. We would drill the center of the top, tap it and then put the knob, nut and threaded rod combo together. The lightest spring we could get was put in the canister and a touch of red locktite helped keep the knob and threaded rod together. The nut of course was to lock the adjustment in. We would take hits and adjust the secondary until it no longer bogged. Honestly unless you have some of this stuff already, the piece from QuickFuel is less hassle.