Too much power too quickly with 8 barrels !

[Luke427]

I looked into this over the week-end, and there are two factors:
1) the linkage isn't linear and let secondaries open suddenly at about 50% open throttle.
2) secondaries are vacuum actuated and don't open before somewhere between 3000 to 3700 rpm (hard to measure exactly, used a gopro and iphone to film then synchronized begin of video which might result into up to a good quarter of a second gap). They open almost instantly.

So now I have some thoughts and questions, which turns into a different topic:
- Is it normal that secondaries open only so high in RPM?
- Are they supposed to open instantly? I'd say no if both carburetors didn't act exactly the same, however they are almost perfectly synchronized.
- the recurved distributor kicks the advance around 3500 RPM, which is close to where secondaries open, and accentuates the uncontrollable aspect of the car around 4000 rpm. I have been surprised a couple times myself. Does this make sense to consider the spark advance curve? That said, I'll treat the advance separately once what I believe to be a problem with carbs has been solved.
- looks like the linkage can be modified to retain secondaries longer (until almost WOT) however I'm surprised that Holley's website doesn't offer alternative to the stock secondary linkage. Am I missing some parts?

Thanks!
Luke

[Gaz64]

Hi Luke,

If the secondarys don't start to open until 3000 rpm plus, and then open instantly, you certainly have a problem.

Vacuum secondary carbs don't open at that rate.

Starting at 3000, they should open gently and not wide open until about 7000.

Can you post some photos of your carbs and linkage please?

Fix the opening rate of the carbs and you will fix the dangerous concern you have.

Gary

[Racer_X]

Are you running a MSD or other electronic ignition ? Just put in a low RPM limited chip, like 3500RPM. Problem solved. Pull it out when you drive.

[Luke427]

Thanks Gary,
Thanks for confirming - that's what I though as well, otherwise the point of vacuum controlled secondaries doesn't make sense.

Here a zoom on the linkage. They're identical on both carbs:


Focus on a diaphragm/vacuum actuator:


I guess diaphragms need to be serviced/re-calibrated or at least spring replaced? I have never worked on Holley with vacuum secondaries so will have to start understanding them

Ignore the crushed air filters, they have been unfortunately damaged while I was working on the car last week-end.

[Texasdoc]

Put a heavier spring in the vacuum advance pod. You can get a quick change kit to make it easy. I think quick fuel makes a kit that removes that pod and replaces it with an easily accessible screw. It won't snap open and give that sudden boost.

Even with a heavier spring, you will still open the secondaries. Still doesn't fix your original problem.

[BossCER]

The most effective solution would obviously be to put a chevy in it!

[Tim7139]

We have 3 dozen posts here but the root of the problem is simply not equipment. These are all high power, lightweight cars....this we know.
You, or somebody else needs to get the Mrs. Out in a big, non scary environment, like a parking lot and spend a fair bit of time until she comes to terms wth pedal modulation. Barring that you're options are likely two.....another car, or she hurts herself or somebody else.
This is the same stuff as folks going to school to learn threshold braking, wet pavement counter steering, etc., etc.

[Luke427]

Thanks folks.
I need to manage a couple more urgent issues with another car then I'll get back to the Cobra, and will fix these carbs

[Mother]

Use a throttle stop. The lady will get use to the carb set-up