427_Roadster

Hey Gang,

Not sure if this is right or not. Is the linkage (A) the only linkage that opens the secondaries or is there possibly an internal link I'm missing?
Reason I ask is when you slide rod (B) that comes from the diaphram (C) the slotted hole in linkage (A) is elongated enough that the rod can move it's full travel without moving the linkage. It seems as though the rod should be seated in a hole in the linkage instead of a slot so that any rod movement translates to linkage movement. I'm assuming this is the way vacuum opens the secondaries.
Hoping this makes some sense to someone because it confused me just to type it! :crazy:
:cheers:
-Tom

LT1driver

That is the correct rod to open the secs. It is vac. If you are not opening fast enough or to slow, there is a set screw which controls the tension on the opening to prevent bog etc. What problem do you have? :cheers: A good carb often badmouthed for wrong reasons. I actually prefer it over holley on street/strip cars.

Barry's70LT1

427_Roadster...
This is my understanding of how this works.
When the engine is running "C" (dashpot) has vacuum applied to it.
The diaphram in it draws "B" (rod) toward the front of engine, limiting
in "A" (slot). Keeps top butterfly closed.

When you apply full throttle (mash the gas), the vacuum from the engine is actually what opens the top butterflys (flaps).
They were being held closed by "C", (and a spring). Since you are now at
WOT (wide open throttle), there is no vacuum to hold "C" closed. It then
slowly opens and provides a controlled release of the top butterflys.
Without "C" the butterflys would simply flap open and you have the big bog.

As LT1driver stated there is a spring adjustment to control how fast the
butterflys open. Some people have removed "C" and tightened the spring
to get a quicker open. Maybe good for racing, not too good for street use.

Again, this is my understanding of this. Lars knows for sure.

Barry

marky mark

Great pics of carb by the way. Yes, I'll agree that is my understanding of how the secondaries open. Sounds like you need an adjustment of some sort. Hopefully Lars will see this post to help you out. I also wonder what role the choke may play (if any) in how well the secondaries open?

Lars, I'll be talking w/you soon about working on one of my 2 sick Quadrajets. I'm finally ready.

Regards, Mark :flag :chevy


[Modified by marky mark, 2:15 AM 12/2/2001]

427_Roadster

Yeah, I'm ok with the concept of how the diaphram moves the rod to open the secondaries - The real question or problem is that when I move the rod (B) by hand, full travel either way, it does not open the secondaries.
Basically, I think the rod needs to engage linkage (A) to open them but to me it looks like the rod's entire travel is taken up in the elongated slot of (A) without actually moving the linkage itself.
I guess I'm kinda wondering if you moved the rod by hand should the secondaries open? I figure if moving the rod by hand won't open them, then the vacuum diaphram is not doing it either.
Thanks... :cheers:
Tom

lou

Tom, That rod i no way opens the secondaries. Its purpose is to keep them from openning or openning to fast. Pull the rod full foward and notice you can't get the air valve to open(push it open with your finger). Now release the rod and you will notice you can open them with your fingers. What actually opens the air valve (top sec plates) is the actually vacuum from the intake when the bottom sec butterflies open. When this happens, the actuator looses vacuum and release the rod allowing the air valve to open at a determined rate. Hope this clear things up. lou

427_Roadster

:smash: :smash: :smash:
Ok, that makes sense now. Thanks for clearing it up for me. :cheers:
-Tom

lars

Tom -
Just got on the Forum for the weekend and caught your post.

Correct as the guys have been saying. But let me do my usual rambling on the subject.... :)

The Q-Jet is actually not a vacuum secondary carb in the true sense of the word. A vacuum secondary carb typically uses venturi vacuum from the primary venturis (this is different from manifold vacuum or ported vacuum) to sense primary airflow and to actually "pull" the secondaries open.

The Q-Jet is strictly a mechanical secondary carb. The actual secondary throttle plates (down low in the throttle body) are pulled open mechanically the the progressive primary throttle linkage. If you put your foot to the metal, Q-Jet secondaries will open.

To prevent a massive bog, and to keep the car from falling on its face, when you put your foot into it, a secondary airvalve is used. It's the airvalve that you see at the top of the carb, and this is what the rod is attached to in the slotted bracket. This airvalve is held closed by spring tension, and this tension is adjustable. When the secondary throttle plates are mechanically opened, the low pressure area in the manifold will "pull" the secondary airvalve open as far as necessary to affect good performance. If there is not enough "pull" in the manofold to open the airvalve, it will not open against the spring pressure.

In addition to the spring pressure, the airvalve is connected to the choke pulloff diaphragm as you've observed. This pulloff holds the airvalve firmly closed when the engine is running normally (the diaphragm is connected to manifold vacuum). When manifold vacuum is lost, indicating that you got your foot into the secondaries, the choke pulloff has a little air bleed hole in it that makes it "relax" and extend at a slow, controlled rate. This keeps the secondaries from "slamming" open due to immediate engine demand. Since the Q-Jet has no accel pump on the secondary side, an immediate opening of the secondaries would induce a lean-out condition, and the car would fall on its face, in spite of the spring windup on the airvlave. The choke pulloff, thus, does not open the secondaries: it controlls the opening rate by allowing the airflow to force the airvalve open at a controlled rate.

You can adjust the opening rate of the secondary airvalve by playing with the windup spring. On our racecars, we also drill out the air bleed hole in the choke pulloff. I don't recommend doing this on a street car. But here is the procedure for the secondary airvalve spring adjustment:

The secondary spring windup is adjusted with a small, slotted-head screw on the passenger side of the carb, right at the top of the carb on the secondary side. The screw head points right out to the side. 90 degrees from this, on the bottom, there is an allen-head lock screw that keeps the slotted screw from turning. If you have trouble seeing it, place a mirror under the area until you spot it. With a small slotted screwdriver holding the adjustment screw, loosen the allen screw about ¼ turn. This will allow you to turn the slotted adjustment screw. Counting the turns, allow the slotted screw to slowly unwind until all spring tension is gone. You can use your mirror to see the spring disengage contact from the pin lever underneath the air horn. If the spring tension was lost after only ½ turn, the windup was too loose. Bring the spring into contact with the lever. Note when it just barely touches. From this point, wind the spring up between ¾ turn and 7/8 turn. This is a good starting point, and will prevent any bogs or hesitations due to premature secondary opening.