SCOODY

Hi,
On my thirdgen Z28 1984 , 5L , my secondary works , when motor is hot it's ok.
But to have it giving the power, the only way i succeed is when motor is hot (of course) then car is stopped, then i kick down the Accelerator fast until the floor.
In that case only, the "full" power is here. if car is running, then kick the Accelerator , i don't have the full power.

How can i adjust this ?

Thanks for your help.

paulo57509

I "think" this is what you're referring to....

The Quadrajet is designed to prevent the secondary throttle plates from opening until the choke fully opens.

This shows the secondaries "locked-out" when the choke is closed. When the choke opens, the linkage moves out of the way allowing the secondary throttle blades to open.



I don't think there's an adjustment for this.

sofakingdom

iTrader: (1)

Correct, no adjustment... if the choke is on at all, the secondaries are blocked from opening.

The secondaries themselves DO have an adjustment however. it's that little screw just above and to the left of Paulo's red circle, in line with the air valve shaft. Underneath that, behind that black linkage, is a set screw, which is an Allen head, 3/32" IIRC but I could be wrong. Whatever it is, you loosen it, and then you can turn the screw. The screw sets the tension on a wind-up spring that controls the valve.

Most people think that letting out the tension, so that the valve opens easier, is good; but in reality that's the easiest way I know to get that "bog" at low RPMs where the engine makes a whole bunch of noise but no power. It should NOT sound like BUUUUUUUUHHHHH-WAAAAAAAHHHH when you floor it; it should accelerate crisply without hesitation. The BUUUUUUUUHHHHH part of the sound is the 'bog". Its setting is almost entirely dependent on the engine size, and independent of ANYTHING else. For a 305 it usually wants to be between 3/4 and 7/8 turn of tension.

Loosen the set screw, let the spring tension all the way out, turn the screw counter-clockwise until tension can just barely be felt in the blades, then turn it 3/4 turn more, and tighten the set screw. Tune from there for best results. If it has the "bog" (lean), increase the tension. If it blows out black smoke as the secondaries just start to open (rich), reduce the tension.

The air valve also moves metering rods that alter the mixture as the valve opens; and the rods are suspended by a hanger that determines how far into the jet the rods go, and the point where full richness is reached. That vacuum diaphragm on the side of the carb, which is primarily the choke pull-off, is also the damper for the secondaries, and controls the rate at which they open. All of these are further tuning aids. But that's usually not necessary or productive for a stock engine. Setting the AV tension correctly IS critical however.

paulo57509

Here's the air valve adjustment. The air valve opens due to manifold vacuum (air flow) through the secondaries and works against spring tension. The adjustment is to vary the tension in the spring which keeps the air valve closed; adjustment either quickens or delays its opening when the secondary throttle blades open.



In my opinion, unless you want an extended exercise in trial and error frustration, leave this adjustment alone.

SCOODY

Hi, Yes i learned the existence of blocking system on carb when solved my troiuble with choke.
But as i wrote, my "tests" are when motor is hot , and i also wrote that secondary works. My "only" trouble is that to have it works, i have to push to the ground the Accelerator when car is stopped, if i push to the ground when driving it doens't works.

I understood that it coul'd be worst that better to adjust, but thanks to show me where it was !!

So, the real question before taking risks is : "Do you succeed with you car to engage secondery while Driving ?"

sofakingdom

iTrader: (1)

That doesn't make sense.

The carb can't tell whether the car is moving.

Try a borescope in your air cleaner to see what's REALLY going on.

ironwill

???

Something is being lost in the translation here, I believe.

SCOODY

Sorry, I will try to explain better.
I wanted to say that the mechanical choke system that blocks the opening of the secondary is not in question because it works well.

So no troubleshooting to be expected on this side there.

But the only case where I manage to trigger the secondary and have the power is when the car is stopped, engine warm, and I accelerate fully. In this case only the car accelerates strongly with the secondary. If 1km later, I drive at 20km / h and I accelerate fully, I have a simple and not very powerful acceleration.

hope it's clearer

Tadream

Sounds less like the carburetor and more like it's not down-shifting and therefore "lugging" in too high of a gear when he's already moving. TV cable, maybe?