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Discussion Starter · #1 ·
Can anyone explain the vacuum secondaries in my Quadrajet? According to what I got it doesnt make sence. The butterflies on the secondaries are spring loaded closed. The vacuum diaphram, when it has vacuum on it pulls back on the linkage which also closes it. So what opens up the secondaries? Am I missing something?












 

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quadrajet does not have vacuum secondaires. The throttle blades for the secondaries open at a given throttle position, and the spring loaded upper butterflys open against spring pressure when the airflow is there. The vac diaphragm you are talking about is most likely the choke pull off.
 

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Discussion Starter · #3 ·
The diaphram has 2 linkages. 1 goes over to the choke the other goes to the secondary butterflies. I wonder if that is why they wont open, when I stomp it it kinda bogs down like its missing something. Vacuum through the carb throat is what causes the butterflies to open which in turn let more fuel into the bowl right? With the diapram hooked up, it stays in the compressed position holding the butterflies shut. I tried opening them while running could not do it.
 

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I don't know quadrajets very well but at WOT there is no vacuum at that diaphram to hold it closed. There is an adjustment for the spring tension on Secondary air valve
 

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the newer guadra bogs im not to sure about but the older ones(yea) with the motor not running hold it wide open on the left side looking at the carb you should have a rod that opens the bottom secondarys if the lefer that opens the bottom blades is laid back almost even with the carb base they are open. the vacume comes in on the top blades the are spring loaded and wont oopen until the motor pulls enough vac.on the top of the carb there should be a little allen screw, loosen that and that will let you udjust the opening of the sec.you can udjust the bootom blades by bending the u like rod aft(back) towards the fire wall, a little goes a long way here be carefull.i usda have my tops set to were they kinda dangled there you probably coulda spit on them and they would move.if they wont open just by pushing them down hold it wot and try it or its the choke try it with the motor warmed up. that is how the older styles are these new ones,computer control crap :mad: and the ones from edelbrock im not sure of so if this doesnt work for u i will go look at a newer one at a speed shop and figure it out
mike
 

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if the choke isn't adjusted right, it may not allow the top butterflies to open.. On mine, it had a lil kicker coming off the choke that held the rear butterflies locked closed until it fully warmed up... but not any more..LOL Also mine bogged when you punched it, but inside mine, the secondary jets were stuck closed... just a lil cleaning and workin great now.
 

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The reason you can't open the secondaries, is that pull off linkage ALSO keeps the secondaries (butterflies)closed UNTIL vac goes away, then it will let them open with airflow and throttle position. At idle vac is high. This keeps you from opening the secondaries just by revving the engine. The secondaries won't open until the engine needs it..which is signaled by a loss of vacuum. Hence, you can't get them to opne without a large load on the engine, and you won't get that it idle.
 

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Ok, I just went out puled my QQJ off the shelf, and checked.

The vac adv should be hooked to the small tube just above the drivers side idle mixture screw on the front of the carb. The other small tube on the pass side, I have no idea what it's for, and I had it capped off. The large tube on the frontof the carb, pas side is the bowl vent line and should go to the charcoasl canister.

The choke pulloff/dashpot should have a vac line that goes from it to the small tube on the rear bottom of the carb, right by the big tube that your pcv should go to. This small tube should be angled towards the dash pot.

This dashpot has 2 functions:
1) it is your choke pull off, IE it opens the choke if engine demand dictates it's needed before the choke is warmed up and opens on it's own
2) It holds the air valve for the secondaries closed to prevent bogging if the throttle is opened up enough to also open the secondary throttle blades, but there is not a demand yet for them (ie you slowly open the throttle up 2/3 or so)

If I go out and fully open my throttle linakge, it wil lopen the primary and secondary throttle blades all the way, and I can open the secondary air valve with my finger agains tthe spring tension. BUT...if I put vac on the dashpot with my hand pump, I can not openthe air valve, but the throttle blades will still open, cuz they are mechanical. Only the air valve is vac actuated on a QJet.

Does this (along with looking at the pics I told you to look at) help to answer your questions ??


The secondary throttle baldes are opened by the throttle linkage, and they are mechanical. The air valve, of top butterflies, are what is opened when the deman is there. They are held shut by vac thru this line. When vac goes away, then the dashpot can no lnger hold them closed, and it allows them to open.
 
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