I have a Holley 670 as well on a Ford 306 crate motor. I went to long yellow spring and then back to purple. For drag strip I would think soft spring, full throttle and there would be no bog in a 1/4 mile. There is also interplay with pump cam and nozzle as well as jet sizes. It's a balancing act for general use. More is better for drag racing. Why not use your run times and change out spring between runs? (assuming everything else stays the same)

How tight should the zip tie be and why wouldn't the secondary shaft spin in both open and close directions and not move the zip tie at all, Or move it when it opens and move it back when they close.

The zip tie should be "handshake tight." That means tight enough to stay put on a rotating shaft, but loose enough that the shaft can rotate within it. When the secondaries open, the shaft rotates in a counter-clockwise direction. The zip tie rotates normally along with the shaft, but it is stopped by the throttle screw, so the shaft continues to rotate "inside the zip tie loop." But when the secondaries close, the shaft just rotates back clockwise and the zip tie stays fastened to the shaft in the exact same position. The zip tie will always register the maximum amount that the secondaries opened during a test run.

The nice part about a Holly with vacuum secondaries is that the secondaries only open after you have sufficient air flow through the primaries. When adjusted properly you should not have any bog during acceleration and they only operate when the engine demands more air flow than the primaries can provide. As a result, they are more economical on fuel than a Holly with mechanical secondaries. Under WOT acceleration, if you install a small screw in the slot of the mechanical linkage between the primary and secondary shaft, you will assist the opening of the secondaries while the AFR is still slightly rich due to the accelerator pump shot. This is why a four barrel with vacuum secondaries are more suitable for street applications.

Thanks that makes perfect sense!!

Thanks - I had it in my mind you were fastening it around the vacuum canister rod so that the tie slid along the shaft as the rod was sucked up into the canister. I'll give it a try.

Short of all-out 100% racing, vacuum secondaries are, in every way, superior to mechanical secondaries. Their only downsides are you have to get them adjusted just right for full enjoyment. Just remember, the vacuum is venturi vacuum -- and the only way you create that is by hard acceleration. Chas' posting above is partly in jest, but more truth than not. In order to really set it up properly you have to get on it as hard as you can. When you do, there should be no detectable bog, you shouldn't be able to "feel" anything, and when you check your zip tie, it should show that everything opened up as far as they will possibly go. Unless you have an airport runway nearby, it's a rather dangerous test.:cool:

No, not jesting. Actually, one exit on the parkway or the luxury of the beach road on a weekday was a frequent and easy way to get 'load'.
Did it weekly on the Connecting Highway or 231 as a callow (but talented) youth.:cool:
Certainly the dragstrip or autox course is more ideal. Hell, Bruce in 3170 gets that in first gear but I'm sure he does not have nancy vacuum secondaries...:LOL:

I used the zip tie and it showed a 45 deg rotation. I was under full throttle in third gear at about 3800 rpm. Is this in line with expectation? Under what condition should I see a full 100+ deg rotation. I had no bog and currently am running the long yellow secondary spring. I could only punch it for a few seconds since the road was short.

No, that doesn't sound like they're opening fully. Here's how you tell. With the engine cool, use some string to tie your throttle back as if you were flooring the accelerator pedal. Then, push the zip tie's tail over so it's touching the throttle screw or whatever else is over there to stop it. Next, put your finger over on the bottom of the vacuum secondary plunger rod and push it gently all the way up in to the secondary housing, thus opening your secondaries fully. Take your finger off the rod and the secondaries close. View the position of the zip tie tail -- that is what fully opened secondaries look like. Now remove the string.

Say what? Even Holley and Edelbrock don't recommend a vacuum secondary carb for our light weight cars!

Only because the secondary spring rate is more difficult to nail on a light weight car. But once you've done it right, it's superior.

The zip tie trick works great. I tried it today.
Thanks patrickt.

I respect patrickt's experienced opinion on preferring the vac sec. for the street. I've had both and other than utmost efficiency in smoothness, i like the mechanical. I'm more in line with having the feel and control at my fingertips or foot, I know exactly when the secondaries are cracked, I can feel it as well as hear it, I can control it. Just like I would not put an auto in my car or soft body mounts, or power steering, or engine mounts, my car is all about, sound, feel and quick action. It's personal I know, but I did not feel any disadvantage at all going to mechanical. It keeps me busy :) These cars can be a handful or a footfull, it's the way I like it

That was a good post. Here's the problem with vacuum secondary carbs and Cobras: In a light weight car, like a Cobra, it is easier, and safer, to have the secondaries not open as quickly as they should, because, if you go just a little too far past the secondary opening "perfect sweet spot," then the onslaught of acceleration from the secondaries flopping open can be unpredictable, break the wheels loose when they shouldn't, and detract from the overall enjoyment of your car because the acceleration resulting from your foot movement is not consistent . Finding the sweet spot is not always easy because, on a very light car, like the Cobra, one spring might not open the secondaries fully, and the next spring will flop them open too quickly. When this is the case, your choice is to either switch to mechanicals or live with them not opening as efficiently as they could if you had been able to find the "perfect spring" (which you can't). The solution is to have an adjustable vacuum secondary that you can dial in with the turn of a screw (instead of changing out a spring), coupled with a simple, easy to tell method of how far your secondaries are opening. Short of doing this, you have to either "get lucky" with one of the incremental springs being just perfect for you, or you live with the secondaries not running to their absolute full capacity, or you switch to mechanicals. Truth be told, on a street driven Cobra, we have so much power that there is actually very little true difference between a "perfect sweet spot spring" and the spring before it, that it doesn't really matter all that much. And, in fact, before I put on my adjustable vacuum secondary housing, so that I could find that theoretical perfect sweet spot, that is exactly what I did.

I just completed this process and finally got the secondaries to open properly. I have a 351W, 380HP, with a Holley Model 3310 750 cfm. The carb had the stiffest spring (black) installed from the factory. After trying the medium (plain) spring, and then the light (short yellow) spring with no change, I finally ended up with the lightest spring in the kit (white). Even with the lightest spring, the secondaries did not open on first try. I suspected the tie wrap may be too tight on the shaft (inhibiting actuation), so I removed it and used a paper clip, as recommended on the Holley website (installed on the right side vacuum actuator rod). That resulted in an obvious improvement in performance and the paper clip had moved, indicating proper actuation. I think I used too wide a tie wrap and too tight the first few times. Now have a smooth rush of power, with no noticeable bog. I did not remove the check ball, but I did enlarge the bypass groove slightly, as the factory groove looked very small and I wanted more vacuum signal to pass through. Thanks to patrickt and the rest of the commentators for this very informative and useful thread. I can now expect a reduction in the 13 mpg I have been getting up til now. This engine would probably be happier, and probably better performing with a 650 cfm carb. I'll save that for another time.