Bobchad

Was tinkering with the car last weekend when I noticed that the secondaries are not opening on the new Q'Jet. So now I need to play with that.

Can someone give me some advice on what I am looking for and how to adjust?

I've added dual exhaust, re-curved the distributor and added the Edelbrock Performer with what I would call slight improvement. Hopefully this will open up the potential that others seem to have experienced with the same mods.

Larry B.

The secondaries will not open without enough engine load. You cannot make them open while you watch. You can adjust the opening rate. Go to http://www.corvettefaq.com and find Lars Q-jet tunning paper.. Great info. Also there is a lock-out that prevents them from opening until the choke fully opens. Also ... when you say "new" do you mean "new " or "rebuilt"?

isosceles

I can get mine to open while revving it in the drive way, but only very slightly. It only does it with a very quick revs to 4K+ rpm. You might think the windup spring is too loose, but I have no bog like I used to when they open.
Two things especially to take note of in Lars's paper is the secondary windup spring tension and the part about the flaps binding due to the top of the carb being tightened too much and distorted.

As Bluevetteman indicated, they won't open when locked out by the choke. Make sure the car is warmed up before you try to get them to open.

Diode

Guys:
Here's the thread about adjusting the secondaries - http://forums.corvetteforum.com/zerothread?id=609372
I was just messing with this adjustment today. Made the motor slightly more responsive when I backed off on the spring tension ever so slightly. Surprisingly enough, the secondaries don't "snap" open when you hit the throttle. Looking at them with the car parked in the driveway, I had to really nail it and get the revs up for them to open and they did that relatively slowly. As said above, the car has to be under load for the secondaries to operate properly.

Larry B.

You will get the Q-jet "bog " if they open too soon. You should not feel them open because they should open gradually and smoothly as needed.

Bobchad

I understood it to be new. Thanks for the info guys. I definitely have the "quadra-bog" and with this info I'm concerned that perhaps they are opening too early.



[Modified by Bobchad, 10:03 AM 8/3/2003]

Diode

I've got a problem with that. How do you get neck snapping 0-60 times with the secondaries opening gradually and smoothly???

Larry B.

It's sort of co-ordinated on demand system. Many things determine what that will be. Q-jets have huge secondaries so they can't help bogging if they open all at once. That's why there are so many different Q-jets listed by GM. This is also why you must tailor any carburetor to you engine. The Idea is to control the fuel air for the best performance, not just dump a bunch in all at once. More is not alway better. Carbs with mechanical secodaries do just that. While it may feel neat it may not be the quickest. "Seat of the pants "feel is often mis-leading.


[Modified by bluevetteman, 10:23 AM 8/3/2003]

Diode

Even though what you're saying makes absolutely perfect sense, I prefer the feel of "explosive" power. I keep comparing my '68 SB to my friend's '67 SB (a mistake, I know). His car came with the Holley double pumper w/ mechanical secondaries sitting on the aluminum high-rise manifold. His car will leave 30 feet of rubber in second gear. Granted, he's also running 3.90 gears in the rear :). I grew up in the era of Boss Mustangs, hot GTOs, Charger R/Ts, etc. That's the kind of performance I always lusted after and couldn't afford. Now that I'm 49 years old, I finally got my muscle car (sort of). True, this car is more about looks than performance, but I would still like to get that kick-in-the-butt feel that I know it's capable of. I may wind up taking off the original equipment and putting it in the attic and putting on an Edelbrock intake and Edelbrock or Holley carb. To me, the vacuum operated secondaries are the first, albeit primitive, step towards "drive by wire". BTW – I've been searching everywhere and I can't find the CFM rating on the factory carb…any idea where I can find this information?

Larry B.

Q-jets are 750-780cfm depending on the year... The later models are the bigger ones. There are a few odd Q-jets that are bigger. but not on Corvettes. Believe it or not one ford engine in a cobra used a Q-jet. Also ... engine tune and fuel have a lot to do with the power. If you set the timing back or limit the total advance so you can use the 93 octane fuel of today the engine will not produce anywhere it's potential.