Hey guys...need some expertise here. 4bbl quadrajet. The pic below is at normal operating temp. Are the back flaps just not supposed to do anything? Maybe only at WOT? I dunno. I feel like I should be getting a little more out of the car so when I noticed these plates being closed I was curious. Idle and timing and everything else is smooth as butter. She cruises great..just looking for your thoughts.
Thank you!
[/IMG]

 

Call the Man behind The Curtain..Greg Kalkhoff Sparkys Carbs in Milwaukee.

 

quadrajet.jpg

The back plates are the secondary air flaps.
They are opened by air pressure when the secondaries are activated.
The air opens them enough to allow enough air in to quench the vacuum produced by the engine, and as they open, they lift the secondary metering rods, which enrich the secondary mixture appropriately.

You should never see them open if you have a hood on your car, as it is not possible to get the engine to take in that much air at idle.
If you drive with your hood and air cleaner off, you will see them open.

- Eric

 

The rear butterfly valves in your carb are your secondaries. They are vacuum operated and open when the car demands more air and fuel. At idle they will be closed. If you have the air cleaner off and run the throttle wide open while running, you should be able to see them start to open when the engine hits higher rpms.

 

MD, you got there just a second ahead of me!

 

thank you guys. great explanations. have one more pic and question. gotta go shoot the pic now. glad you got that pic somehow. my wifes computer sucks.

 

 

http://http://s48.photobucket.com/al...rent=carb2.jpg

 

I can't give you a clear answer, since you're using a Chebby carb on an Olds engine.

In general, though, the vacuum break diaphragm is run from a dedicated port on the carb, with a single, dedicated, short piece of hose, with nothing else attached to it. As I say, in your case, you seem to be running a post-1977 Chebby carb in a pre-1970 Olds, so the original arrangement would probably not apply anyway.

carb2.jpg

- Eric

 

Just to be completely correct, they are neither the "secondaries" nor are they vacuum operated. The secondary throttle plates are on the bottom of the carb and are what open when you mash the throttle. The plates on top are called air valves and as noted, they open only enough to satisfy the airflow needs of the engine, based on both the amount of opening of the secondary throttle plates below them and on the RPM and load of the engine at the moment.

Contrary to popular belief, these air valves are not vacuum operated. Holley used vacuum operated secondary throttle blades where there is an actual vacuum diaphragm that is connected to engine vacuum - this diaphragm actuates the throttle valves. The Qjet has fully mechanical secondary throttle valves. The air valves are actually operated by the mass flow of the air entering the engine - consider this to be a mechanical version of the mass air flow sensor on EFI cars.

The difference is that on a Holley, if you were to hold your hand over the top of the carb (or otherwise completely block airflow to the secondaries) and then open the throttle with the engine running, the vacuum diaphram would still open the secondaries even with no airflow through them. Do the same thing on a Qjet and the lack of airflow would mean that the air valves would not open. They are airflow controlled, not vacuum operated. Carter (now Edelbrock) carbs work the same way.

 

True, they are not vacuum operated, but they are vacuum controlled (or lack of vacuum controlled) and air flow operated . The small diaphragm on the side of the carb controls when the air valves are allowed to open. This releases when the pressure inside the manifold drops toward atmoshperic. The adjustment screw allows you to determine how soon you want the valves to open.

 

OK, fair enough. Unlike the Holley, the Qjet diaphragm controls the RATE of air valve opening. The opening force itself is still provided by the mass airflow through the secondaries. Interestingly, the original Qjets didn't use a vacuum diaphram but instead used a fluid-damped dashpot to control airvalve opening rate. This dashpot was a piston that used fuel in the fuel bowl as the hydraulic fluid.

Note the arm above the driver's side air valve on this 1966 Qjet. The forward end of this arm (left end in the photo) attaches to the piston that is down in the fuel bowl.



By the way, the adjustability you mention is exactly why I like the Qjet. You can change the air valve opening point with the wrap spring and you can change the opening rate with the diaphragm. Since the air valves are connected to the secondary metering rods with a cam, you can also change the rate at which the secondary side richens up based on mass air flow (just like an EFI system!).

 

I never did understand why so many people dislike the quadrajet. It really is a very adjustable and tuneable carb. You just need to take the time to learn how it works and how to properly set it up.

 

OK, I was fine up until then. Compared to Holley and others, it is much more difficult to tune. I can adjust the pump shot, rate, and volume in ten minutes, how do you do that on a Q-jet? How about float level and primary jets? I DO agree that a properly set up Q-jet is an awsome street carb, but to characterize it as "very adjustable and tuneable" is not accurrate, IMO.

 

I did not say that it was fast to tune it nor did I say that it was incredibly easy, but rather that its not the horrible lump of a carb many make it out to be. You said yourself that it is an awesome carb when set up properly. If it is not tuneable or adjustable, then how could you possibly set it up properly?

 

I'm not going to get into a big arguement, I just think it is more difficult than you make it out to be, especially compared to other carbs. And again, mose Q-jet experts suggest DRILLING is the best way to tune the pump shot. What happens if it is too much?

 

Jim, before you reply again, please read the post and understand what is being said. No where did I ever say anything about it being EASY. As a matter of fact, I did say you need to take the time to learn how to properly adjust and work on these carbs. This actually implies that it is difficult and requires study and practice, not WHAM BAM THANK YOU MA'AM your carb is adjusted. And about over-sizing the hole, that shows you were trying to rush and not step up in small amount to reach the proper size. I guess that why Holleys are great for people with no patience. In the future, please don't try to twist and manipulate what I have to say in order to make it better fit your argument.

 

This was YOUR quote, I am not twisting anything. Please tell us how you tune the initial pump shot on a Quadrajet. IMO, by saying VERY ADJUSTABLE AND TUNEABLE you ARE implying that it is easy. To come in afterwards and deny that is, IMO, disingenuous. Thae parts alone are much more difficult to round up. Is a guy supposed to by a gazillion hangers, rods, and jets (if he can find them) until the right combination is found? Yes, the Q-jet gurus who have years of experience and tons of parts laying around can dial one in (although I gave Dean Oliver $400 for a Q-jet that was a POS, ran very lean) but for the average guy tearing the carb apart to adjust the float or change jets does NOt fall into the category of "Very Tuneable" IMHO

 

Holleys are great for guys who want their cars to be fast and easy to tune. You can call it impatience but for performance it is a better design. Better pump shot, larger fuel bowls, easily adjusted secondaries, and on and on.

 

OK Jim, until this point I have tried to be civil with you, but seeing how there is no reasoning with you, I'm done with being nice. In all the post I have seen from you, you are always trying to create an argument or cut somebody down. You will stop at nothing to try to rip into someone, including but not limited to twisting words, bringing up new topics to try and argue, and just outright be a rude SOB to people. Seriously, If I wanted to hear an *******, I'd fart. Due to your outrageous ramblings, I have completely lost any respect for you. Even if in a longshot, you do propose a decent point of view that contains some shred of helpful information, nobody even cares because it is covered by mountains of malicious BS. Most everybody on this site gets along well and tries to encourage and help each other, but since my very first post on this board, you have had nothing but negative, dissenting, egotistical crap to say. Seriously man, why can't you just grow up? I am quite certain that there are MANY people on this site that share my sentiments.

To the OP, I am sorry that your thread came to this, but it had to be said.

 

You are certainly entitled to your opinion and are free to voice it, as am I. I do not insult or name-call, but when I see a statement that I disagree with, or agree with, I say so. You made an incorrect statement, referring to a Q-jet as a vacuum secondary carb, which it isn't, and Joe corrected you. I disagree with your "tune" statement and gave the reasons why, nothing personal. If a guy says you can get a completely rebuilt engine for $2800, I will disagree and state the reason. If a guy says that a 230 @ .050 cam is a good choice for an 8 to 1 350, I will disagree and state the reason.

You are the only one insulting anyone in this thread. Your "patience" remark was not necessary and elicited a like response.

Q-jets are fine carbs and I suggest them 3 to 1 over Holleys. But, they are not easy to work on or to dial in. There are countless threads here on CO from guys who have had trouble with them. If you had stated,

"The Quadrajet is an intricate and sophisticated carb that once dialed in is an excellent choice for a street driven vehicle, giving a nice combination of performance and economy. However, there are infinite combinations of jets, rods, and hangers and each change requires partial disassemby of the carb, making it more difficult to work on than some aftermarket carbs."
I would agree completely.

Don't make incomplete or incorrect statements then get mad at me.

 

Well, I guess that I am sorry I didn't confer with you before my first posting to make sure it was to your liking and satisfaction. I guess in the future I will just write that I am waiting to hear if you will be OK with what I want to say.

 

Say whatever you want, that is the point of these public forums. Most of the things you post I agree with. In this case, I did not. No biggie, if we all thought exactly the same the world would be pretty dull. I disagree with Nick on HP, Mark on Engle cams, etc, doesn't mean either of us is right or wrong or that I don't respect their opinions.

 

Ok, fair enough. And I apologize for getting personal with it. Just got the best of me I guess. I had a rough afternoon putting in a transmission and took my frustrations out in the wrong area. I didn't mean any disrespect and usually don't go off like that. So if you would, just disregaurd what I said when I got personal.
Thanks much.

Tom

 

i find them pretty easy to work on, but maybe that's because i've worked a few hundred of them. lol

and i don't subscribe completely to the non-vacuum secondary theory posted above regarding qj's, but we've already had that debate, and we agreed to disagree.


bill

 

One thing I am NOT going to do is get between you and Joe discussing Q-jets!! LOL

 

Thanks for the info guys. Very helpful !