rktolds

Does anyone have a good pic of their quadrajet on an edelbrock performer intake? My car is a 72 Cutlass. I'm wondering if my vacuum lines aren't in the right spots. It may sound dumb but I don't know why my secondaries won't open. I do have a "catch" in the throttle just after the accelerator pump does it's thing. Meaning it's not a smooth operation to go from idle to full throttle. I've tried to adjust the secondary spring but haven't had any change.
It's a 455 rebuilt in the 90's with J heads and performer intake. It idles great and pulls 14" or so of vacuum at idle. It runs great down the road but I'm pretty convinced I'm only running on two barrels.

Let me know if I'm forgetting details that could help.

joe_padavano

This comes up often. The plates on the top of a Qjet are NOT the secondary throttle plates, they are air valves that only open under load. The throttle plates are on the bottom of the carb and are mechanically operated on a QJet. The air valves on the top of the carb are NOT vacuum operated, they are opened by mass air flow through the secondary side of the carb. You will NOT see them open if you are simply revving the engine in park.

midrange

I have a lot to learn about Quadrajets, (and carbs in general....) but what is it that opens the secondarys? And what is the chain of events that enable/cause them to open.

If I lay into the throttle pretty hard, standing start or rolling, when I get around 4000 rpm, the engine starts missing and cutting out. At that point I lay off the throttle, the rpms come down and the engine picks back up like nothing ever happened.

Would this be a sign that the primaries can't supply enough air/fuel alone, and need the secondaries to help out?

In park, I can rev the engine up to 5500 no problem, but, there is no load on it, so I'm assuming that is why the primaries can handle it alone.

brown7373

Could be a couple things. As Joe said, the throttle plates are down low, under the air valves on top. When the throttle is opened sufficiently, the lower throttle plates will start to open and will be fully open when the throttle is fully opened. The incoming air under load will pull the air valves open. No fuel flows until the upper air valves open.


Several things can prevent them from opening. If the throttle cable is faulty or the bracket on the rear of the carb is bent, the throttle may never fully open. Also, there is a secondary lockout on the top of the air horn. It has a little tab that extends over the edge of one of the air valves to block it from opening. This pulls back after the engine reaches operating temperature. But it can be misadjusted so it never pulls open. I usually remove them on my carbs to eliminate the potential to block the air valves. Just don't operate the engine at full throttle before it warms up.


If you don't own a Service Manual, buy one. Also, if you want to learn about Quadrajets, buy Cliff Ruggle's book, or Doug Roe's book on Quadrajets. Don't attempt to adjust or modify a carburetor unless you learn about them first.

NHolds

I just grabbed this offline, but this is what opens the secondaries

Picture014_zps40878221.jpg

Fun71

First thing to do is have a buddy sit in the drivers seat and push the pedal to the floor while you look at the drivers side of the carb and verify the secondary throttle blades are opening all the way. As said above, if the throttle bracket on the rear of the carb is not aligned correctly the throttle may not be opening fully.

X2 on the secondary air door lockout tang possibly keeping the secondaries from opening. You can check this with the engine warmed up to operating temp. The primary choke valve should be open enough to move the tang off the secondary air valve and allow it to open. If it's not, then it may be as simple as adjusting the choke.

rktolds

Thanks guys. I will take a look at it this week. I'll put my daughter on gas pedal duty. She'll love it.

Matt

BlackGold

Let's put it this way: the NASCAR boys run in the neighborhood of 200 mph on the superspeedways with their engines screaming at twice your RPM while sipping through restrictor plates with amazingly small holes in them. In other words, I doubt your symptoms have anything to do with the secondaries not opening.

If your engine pulls hard right up to the point where it cuts out, and this only happens at high rpms, I vote for a fuel delivery problem. Could be pick-up, pump, filter, float, lots of stuff. Could also be an ignition problem.

rktolds

image_zpsbf3a71ac.jpg

I think my cable is too long. I thought I ordered a 4 barrel cable years ago. I'm gonna check it with some a buddy has. My daughter mashed the gas and the primaries only opened 1/2 the way. I think I'm on the right track.

rktolds

Any other thoughts? I'm hoping this can help others too.

midrange

Just saw this, so sorry for the late reply.
It was due to the secondaries not opening. There is a bit of a tuning issue I need to address, but I'm about 85-90% there.

I studied a Quadrajet manual to see how the secondaries open. What I found on my carb, was that even when the choke was open, the vacuum lockout wasn't pulling off and allowing them to open. I couldn't find anything that connected them together in order to work as designed.
I removed the vacuum line attached to the canister and plugged it at the carb. Took the car for a spin and hit 5200 rpm.

I knew I still had some work to do, but I was making progress. I looked on the driver side of the carb and found a tab that presses against the rod assembly that opens the lower throttle valves was bent in a manner that even at wide open throttle, the tab never made contact with the rod assembly. I straightened the tab, and took it for another spin.....

Holy smokes!!!! I have a new car. 6500+ with an exhaust roar I never dreamed I would have. The car flat out screams now.
Even when building up to 4200, the engine has a more powerful pull.

Sorry I don't have any pics to clarify what I did right now, but I will take some and get them posted in the morning.

When I shift into 2nd and 3rd I still have a bit of a flattening out at 5000 rpm that the car doesn't recover from without my lifting off the pedal a bit. But when I'm in first, it will pull up to the rev limit of 6500. I'm going to look into a fuel delivery problem when time allows.

joe_padavano

The other thing to check is the air valve spring wrap setting. It might be too loose.

midrange

Thanks Joe. Is there a quick and dirty way to check and adjust it if necessary?

Here is a pic of the vacuum canister I disconnected:
0726141205_resized640x360_zps94d5b4f8.jpg

And this pic, in the red circle, is the tab that was bent. It was twisted to where the green line is, and at 3/4+ throttle, it couldn't press the lower tab (just outside of the red circle on the right) and open the secondary throttle valves:
0726141206_resized640x360painted_zps53ac7334.jpg

I think I'd still like the secondaries to open a bit sooner, but I'm not sure how to make that adjustment.

Seff

The bog you describe sounds like a lean condition at WOT. An easy thing to try is giving the secondary air valve tension spring a quarter turn of tightening. I had the same problem for a long time and this solved the issue.