Quadrajet help needed

jseabolt · May 12, 2012

So my dad's '76 C20 454 trailering special wouldn't hit a lick the other day. So I pulled the fuel filter and the pump works but no fuel was getting past the float valve. Odd because float valves usually stick open and not closed. Well from my experience anyway...

Having worked on Webers may times I decided to pull the top half off the carb. I ended up opening up a can of worms because I found out Quadrajets use metering rods instead of emulsion tubes like Webers do. I wish I had tried blowing compressed air through the fuel filter port if I had to do it over again.

So I pulled the carb and cleaned out all the passages. As of now the truck starts up, idles fine and runs fines except for one issue. The rear secondary still won't open. Even when the choke is fully open.

I don't think the rear secondary has worked on this truck for many years. And wanted to see why. When I bench tested it, it doesn't appear to be working and can't figure out why. Something is preventing the secondary choke from opening.

There is a linkage that runs from the choke kick off module

to the secondary choke. But no matter what the rear choke won't open. I'm pretty sure I have the linkage attached correctly but doesn't appear to do anything. See photo below.

There is also a tab that that prevents the secondary throttle from opening and I don't understand why it's there either.

As a test I installed a zip tie and removed the secondary chokes. But the choke appears to be what pulls the metering rods out of the rear jets so the result is when I open the secondaries, the engine starves for fuel. Like a Weber with a busted accelerator pump.

I pulled a vacuum on the choke kick off module and it works by the way.

Any idea why the secondary won't open on this carb based on this photo?

Seriously thinking about just replacing the carb. I found a new Quadrajet for $188 + core but is there a cheaper alternative like a Holley? Dad says he wants this truck to get better fuel mileage and I said trying to get more MPG out of a 454 is like pissing in the wind.

gmtech4 · May 13, 2012

the secondary is not a choke , ihavent worked on one in awhile lol,

i cant comment on anything else except to take it to a carb guy ,they are still breathing lol

good luck ,its hard on the internet ya know ?

Doug_Scott · May 13, 2012

The tab that is holding the secondary closed is supposed to lifted when the choke is completely open. Make sure the linkage is not bound up, or installed upside down. Go over the linkage on the other side of the carb that you posted. Been a long time since I had a quadrajet apart, but I seem to recall there was some fancy linkage on the drivers side that would allow the throttle to go WOT, even with the choke on, and it would sort of free wheel on the secondary side if the choke was on. Sort of like a spring loaded pivot, or the lever on the secondary was not attached to the shaft, it would allow the linkage to "slip" against a spring when choke was on. Hard to explain, but dirt simple if you can see it in action.

jseabolt · May 13, 2012

The tab that is holding the secondary closed is supposed to lifted when the choke is completely open. Make sure the linkage is not bound up, or installed upside down. Go over the linkage on the other side of the carb that you posted. Been a long time since I had a quadrajet apart, but I seem to recall there was some fancy linkage on the drivers side that would allow the throttle to go WOT, even with the choke on, and it would sort of free wheel on the secondary side if the choke was on. Sort of like a spring loaded pivot, or the lever on the secondary was not attached to the shaft, it would allow the linkage to "slip" against a spring when choke was on. Hard to explain, but dirt simple if you can see it in action.

Here is the pivot piece I was talking about preventing the secondary from opening:

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The only thing allowing this truck to run without starving for fuel under wide open throttle is the secondary choke staying closed. So in theory it's basically just a two barrel as of right now.

The only thing I can see that could possible open the secondary choke is that linkage I posted that is connected to the choke kick off module. The linkage doesn't appear to be bent, it just doesn't seem to do anything. But the odd thing is the choke is what appears to lift the metering rods out of the jets. Weird.

Dad says not to worry about it but I think it needs to be fixed just because . He used to pull a 33 foot camper with this truck since buying it new in 1976 and my parents no longer have a camper or use it to pull anything so it's not critical.

Doug_Scott · May 13, 2012

It is not a secondary choke, it is there to prevent the engine from bogging when you go WOT. It is an air valve, and will use the air flowing through the secondary to open it. You will note that it has counter weights on it. On the Holley carbs they refer to it as vacuum secondaries. Not quite the same thing, but close.

You will not see it open when you whack the throttle open, no where near enough air flow to open those plates. If the secondary throttle plates open when the choke is off, then the air valve will open when you are driving at WOT.

The quadrajet does not have accelerator pump on secondaries. This air valve will restrict flow initially, and the engine will get a richer mixture while that air valve slowly opens. If you think of its operation as being like a choke, it gets easier to understand. You will also notice that air valve is off centre as well.

txab · May 13, 2012

That carb needs some TLC. A good soaking and a fresh kit plus adjustments

markw8 · May 13, 2012

Qjets were real bad about wearing out the throttle shaft bushings too. It causes an irratic idle.

Mark

jseabolt · May 14, 2012

It is not a secondary choke, it is there to prevent the engine from bogging when you go WOT. It is an air valve, and will use the air flowing through the secondary to open it. You will note that it has counter weights on it. On the Holley carbs they refer to it as vacuum secondaries. Not quite the same thing, but close.

You will not see it open when you whack the throttle open, no where near enough air flow to open those plates. If the secondary throttle plates open when the choke is off, then the air valve will open when you are driving at WOT.

The quadrajet does not have accelerator pump on secondaries. This air valve will restrict flow initially, and the engine will get a richer mixture while that air valve slowly opens. If you think of its operation as being like a choke, it gets easier to understand. You will also notice that air valve is off centre as well.

OK, let me get this straight. The secondary butterflies I thought was a choke, I'll refer to it as a quasi-choke, (simliiar to or resembling, but not exactly) is vacuum operated and what opens it is "simply" engine vacuum when the secondary mechanical throttle is open? What confuses me is the only thing attached to the secondary throttle shaft is a rod that connects to the choke kick off and spring tention on the throttle shaft which keeps it closed otherwise.

This air restrictor also activates the jets, or pulls the metering rods out of the jets.

Now as far as the primary throttle shaft, there is a cam attached to it on the PS of the carb. If this cam is in a certain position, it makes contact with the piece with the pensil pointed at it and won't allow the secondary to open. Now if I zip tie this cam shut, the secondary will open. The question is what prevents this cam from not allowing the secondary?

I was trying to bench test this carb to see if the secondaries were opening but unless the cam is zip tied closed it won't open. I think I even tried making sure the choke was fully open and it still won't open.

Unless there is something I'm missing. Does any of this make sense?