Patrick, you've just enlightened me!!

Check my other thread - last post on the 'too much power too quickly with 8 barrels' discussion.
I think I have the opposite problem where secondaries open suddenly resulting in a dangerous behavior (the engine develops 560 hp and everything kicks in around 4000 rpm at about 50% throttle, which is quite a surprise).
I'm going to investigate the secondary opening springs.

thanks again :cool:

Along with my air breathing top, I am running the K&N stubstack and have a Holley 750 with mechanical secondary's. Can't say that I notice any difference with our without the stubstack.
Patrick, what are your thoughts for mechanicals vs. your vacuum in this situation?

The goal of secondaries, be they mechanically operated or vacuum operated, is to provide just the right amount of increased air/fuel at just the right time when the engine is calling for that increase. If you have your mechanicals, or your vacuum, adjusted perfectly, then it doesn't really matter which one you have. Your car will run as perfectly as possible for the condition that you adjusted them for. It's very easy to mis-adjust mechanicals and very easy to mis-adjust vacuum secondaries. If your secondaries don't open when they're needed, you're robbing your engine of performance. If they open too soon, then you get a bog. If they open "unsmoothly," then you get unsmooth performance. In a light weight, powerful car, like a Cobra, the last thing you want is an unexpected blast of power, maybe after a subtle bog. It makes the car more dangerous to drive.

The physical act of adjusting mechanical secondaries is easier than adjusting vacuum secondaries, because you don't have to take the canister apart and pull the spring out. With mechanicals, you can do your adjusting by the side of the road, as you're testing your acceleration and, most importantly, testing the smoothness. I know Holley has a "quick change" kit for their vacuum secondary spring, but I've never used it, and I have my doubts that it's that "quick." Personally, I always drop little springs, and I always rip holes in diaphragms. That's why I bought the Quick Fuel adjustable vacuum canister for my Holley:

http://www.jegs.com/images/photos/700/793/793-63-12.jpg

It makes adjusting the opening of the vacuum secondaries as simple as turning a screw. If you can just avoid dropping the screwdriver, you'll be fine. :D

My personal opinion is that a "perfectly adjusted" vacuum secondary will be preferable to a mechanical secondary, especially in a light car like the Cobra. It will give the engine all the fuel/air it can handle, and it will do it nice and smoothly, so you don't get any unexpected surprises. Here, all I do is turn the screw, circled in yellow, and I can control how quickly (too much creates a bog) the secondaries open, or how slowly (too little and I'm leaving power on the table) they open. It does not take long to find the spot that is "just right." Anyone can do it, even RodKnock.:cool:


http://www.clubcobra.com/photopost/data/500/qft001.jpg

Patrick this is awesome, I know what to do to fix my brute. I'm exactly in the situation where secondaries open suddenly after a subtle bog, making the car dangerous.

Looks like a pair of Quick Fuel canisters is going to be on my next purchase for the car!

And if you can't find adjustable QF canisters that fits your Holley carbs, just go for their Quick Change kits. I saw your thread on trying to slow her down a bit. Some stiffer vac springs might be "just what the doctor ordered," for around-town driving, at least. With dual quads on there, you'll still have four barrels feeding her even if you have the stiffest "truck spring" controlling your vac secondaries.:cool:

Great info Patrick, thanks. Will have to look into adjusting my mechanical secondaries. I get the subtle bog then BOOM!

You have two accelerator pumps, one for the primaries and another for the mechanical secondaries. If your secondary pump's actuating arm is adjusted wrong and you're not getting a shot of fuel as soon as the secondaries start to open then you'll get an initial bog. The fuel slots in the venturies don't pick up the job until after the accelerator pump has shot its w@d. It's not at all an uncommon problem.

That's why I like my carb. Just one single, simple carb, one single, simple accelerator pump, one single simple vacuum secondary canister. Other than the floats and idle mixture screws, that's it.:cool:

Well, it is not that anyway, it is adjusted correctly. any other thoughts?

Yep, that's a fair point. I was fairly happy with one 750 cfm (but maybe 100 less ponies) :MECOOL: