[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