I've been trying to sort this Quadrajet out. I just put a new nitrofill float, new needle and seat, and new lighter apt spring. It's running pretty good now. I have a vacuum gauge duct taped to my windsheild, and a a/f ratio meter in the console. My a/f ratio is around 15 to 1 at idle (about 13" vacuum), 12.5 to 1 durring acceleration, load, climbing hills, and wot (around 5" vacuum) , and about 16.5 to 1 under no or light load coasting and decelleration (about 15" of vacuum). I have one nagging problem. Whenever I give it enough right foot to engage the secondaries, something sticks when I get off of the gas. The a/f ratio meter stays at 12 to 1, and my idle stays high. After some more driving, or playing with the pedal, it settles down. I thought my apt was sticking in the up position, but I've confirmed that this was not happening. I thought maybe my secondary air valve was sticking open, but that's not it either. The last time it happened, I pulled over, put it in park, opened the hood and took a look at the carb. The rpms climb to over 2500 rpm, the a/f ratio is pegged at 12 to 1 and everything looks normal. The apt was down and the secondary air valve was being held closed by the vacuum pot) It only happens after I've engaged the secondaries. I can drive all day on the primaries and it will never happen. But when I get into the secondaries for just a little while, it takes a while for the rpms and a/f ratio to settle down to normal. What can it be?
Some more info, this q-jet is really modified. It has a two barrel rochester accel plunger, rods drilled, fuel well holes enlarged, larger primary bores and who knows what else. It's worked fine over the years and I've never had this problem before.

 

It almost sounds like the secondary throttle is binding. You should take a picture of the linkage side of the carburetor with the engine idling. Then when it sticks take another one from the exact same spot and compare them. This could tell you what is binding. Make sure the secondary lockout isn't binding. Also, the idle A/F ratio should be about 14/1.

BigBlockk

Later.....

 

So what your saying is that the secondary throttle might be sticking open some, and causing my problem even if the air plates are closed above the throttle?
Merry Christmas BTW.

 

That's correct.

BigBlockk

Later.....

 

I drove the car today. The wife needed brown sugar for the ham and I picked up a dozen roses for the table. Just before getting home, I hammered it, redlined first, redlined second. Afterewards, the idle was badk up at 2500 rpm, and the a/f ratio was pegged at 12 to 1. I drove it home, put it in park, and took these pictures. After shutting it down, the secondary blades seem closed all the way. Maybe you can make something out of the pictures. Pictures at normal idle will be up later.

 

Well you can see that the secondary throttles are stuck open. The secondary air valve is cracked open. This is why the idle is high. The engine is getting too much air.

BigBlockk

Later.....

 

Your casting is distorted around the rear screws, causing the secondary airvalve to stick. File the casting to fix it. However, this by itself will not cause the high idle. Check the actual secondary throttle plates and see if they're really closing all the way - you will get a high lean idle if the actual throttle blades are not fully closing, and this is a common problem. But you also need to fix that sticky airvalve problem...

 

I don't see that. The throttle blades are closed, and the air valves look the same here with the engine off. I also worked the secondary blades, and they seem to close every time. But, you may be very right, because it only happens when I get into the secondaries. If it is sticking open slightly, why? Everything in the linkage seems to be working smoothly. It seems that as soon as I get into a really high vacuum situation, or play with the throttle a little agresively, the idle and a/f ratio return to normal.
Engine off pics:

 

Do you know where the carburetor ID number is? If so, look at the odd shaped linkage that is blocking part of the number in the first picture you posted earlier today. Now compare it to the idling picture you just posted. Do you see the gap in the linkage in the first picture?

Also in the fast idle picture do you see the secondary air valve (the thin metal flaps over the secondary throttle) is open a little?

This is an unwanted air path when the engine is idling. You need to find out why the secondary throttle is sticking. The housing around the shaft may be worn but I would be surprised if that were the case. I would look first at the secondary throttle blade screws being loose.

BigBlockk

Later.....

 

I CAN SEE, said the blind man. You are right. The secondary throttle must be open a smidgen. I can't get it to duplicate in my driveway. It only happens while driving. Great idea of yours to take pictures. I'm going home to Texas and Laouisiana after the Cowboy game. I'll look at the carb when I get back New Years. Thanks Big Blockk.

 

Lars,I didn't see your post till tonight. Wherea do I file? On the base of the carb? I'm not getting a high lean idle, but a rich high idle. I can't understand why the throttle sticks only with the engine running. I will inspect closely when I get home, and file where you tell me. I may just wait untill you come out in February.

 

In post #5 you have the idle mixture at 21/1. This is way lean. Normal idle should be about 14/1.

The area around the rear air horn screws can become destroted if the screws are too tight. Look on the casting for a rub mark and file it out. Make sure the screws are not overly tight.

BigBlockk

Later.....

 

My bad, I fat fingered. When the secondary throttle sticks open, the revs go to 2500 in nuetral, and the a/f ratio pegs at 12to1 rich. I'm on the road now, but as soon as I get back to Cali on the 1st, I will check it out. I thought about it some more last night, and this problem didn't start untill I recently removed and replaced the manifold and carb, so I prolly did overtorque a little. Thanks again.

 

I just got back from my Texas/Laouisiana Christmas vacation. Last night, I took my Q-Jet, cleaned the secondary blades and inspected for where they might be sticking. I reassembled, being very careful to not to overtourque. The secondary blades are operating very freely. I drove the car this morning and it's running beautifully. I kick it so the secondaries get involved, and dangit, it does it again. The idle is raised to around 3000rpm, the A/F ratio is a rich 12 to 1. I leave it running, yank off the air cleaner, and inspect. The secondary throttle is fully closed, so are the air valves above it. In fact I open and close the secondariy blades with no change to the idle speed. What else can it be? This is driving me crazy. It is not flooding like I originally thought was the problem. I invested in a fuel pressure regulator and new needle and seat, but they weren't the culprit. What will make my idle climb and AF ratio go rich only after engaging the secondaries? This is drivng me crazy and cosidering fuel injection really soon.

 

The primary throttle isn't getting stuck on something is it?

As for fuel injection, I plan on doing fuel injection some time in the future. I'm thinking of going with rochesterproducts.com flex-jection TBI. It's E85 compatible and is designed specifically for Corvettes. They give ya a whole kit that comes with everything, so that's right up my alley because I am teh lazy.

 

Nope, the primary blades are closing all the way also.
I like the Accel TBI system. Very simple, and I can control the timing curve, and it's much more affordable than most port injection systems. At around $1300, it's chaeper than some carbs.

 

I think I figured it out. For some reason the APT piston was sticking in the up/ full rich position. I cleaned the piston off with some superfine sandpaper, made it shiney, and put in an even lighter spring with 1.5 coils cut off. Now the rods stay down in light cruise/coast situations, moves up slightly under slight load or trying to maintain speed up highway hills, and comes up all the way for anything more than 1/3 throttle. This carb is pretty amazing as I watch the AF ratio change under each situation. 15 and 16 to 1 Lean at cruise/coast, 14 and 15 to 1 under slight load, and 12 and 12.5 to 1 under acceleration. And it was designed back in the '60s. I think my full rich is perfect, but the cruise/coast is too lean. When I floor it while cruising, I get a slight bog. Can I put a washer on top of the apt adjustment screw to raise the downward limit. It's already screwed five turns up, and I don't dare turn it out anymore. Maybe I should go one jet size larger, but that would make my full rich even richer.
The only thing that has me confused is, why did the apt stick up only when the car was running and only after I gave a good shove on the throttle? I assume that when the engine is not running, the apt comes all the way up, and settles down when the engine starts and generates vacuum. But it never stuck in the up position when I re-started the car.
Anyway, I'm glad I finnaly got that figured out and the car is running pretty good now. I invested in a fuel pressure regulator, new needle and seat, and new nitrofill float, because I thought I had a slight flooding problem. I'll leave it alone till Lars can look at it. Thanks Lars, Biggblock, and Kalway for your help. On to the hydroboost install.

 

Excellent job on finding that. Ya know for some reason I kept thinking to myself, "I wonder if his power piston is possibly getting stuck in the up position." I guess I shoulda spoke up on that one. Oh well you figured it out though, maybe try a slightly skinnier primary metering rod to richen up the cruise a tad.

 

You are right, a slightly skinnier primary rod would not affect my almost perfect WOT AF ratio, only richen up my idle and cruise. And maybe I can put my apt adustement back to a more realistic one or two turns. If I go down two sizes, I need a 40k rod. Is that a size that is still available?
Man I'm glad I got you guys.

 

Just for interest's sake,
What is your carb number and what jets/rods are you using?