1983Zsled

I've got a 1983 quadrajet that i have "worked over" enlarged idle circuit, ect.
The issue I have is that at very light part throttle the wideband AEM UEGO I have goes up to 17:1 a/f mixture. Sometimes it'll run around 15:1 I have turned out the APT stop to 5 turns now and it does not make a difference where it is. at half throttle it is about 13:1 and reads the same at idle. WOT is about 12.5 :1

I notice it at cruising speeds on the highway the most. I have tried different spring rates in the APT as well with no change. The motor is a 350 .030 over with vortec 906 heads summit K1103 cam Int .442, exh .465 with 1.5 rockers, 700r4. The carb has 43k primary rods, 73 jets, and CP secondary rods.

I have read that a leaking exhaust will cause lean a/f is this the issue?
Any body have any ideas? I may turn out the APT another turn.....

The car runs great and seems to pull well all the way through the rpm range.

1983Zsled

ttt

1983Zsled

I have solved this issue. The AEM UEGO wideband is a great tool for dialing your a/f ratio in. I have adjusted the APT stop. Apparently the Q-jet is designed to run very lean off idle and into very slight throttle, and it is acceptable to have a/f ratio's around 15:1 while cruising. It only stays at 15:1 or so while I feather the gas while cruising.
( a very short time) As soon as you let off the gas it reverts back to 13:1 a/f.
I guess this is why people like the Q-jet. It gives decent cruising economy, and performance when you step on it.

Sonix

iTrader: (1)

Very cool. Can't wait to hook up my wideband to see what's going on...
Normally i'd set the APT at a level where the lean-cruise is on the "fat" part of the primary rod (the .043" part), and leave it. I wouldn't want my lean-cruise to be on the power-curve 'cuz that would be messy. I'd just shrink my rods if need be. That's just me though.

TPIscreamer

Yeah thats the weird thing about q-jets, when dealing with stuff like this happening you have to keep in mind the design of the carb, with the small primaries and huge secondaries. But remember, Q-jets are carbs that you actually have to go at full throttle a few times a month, cause their accelerator pumps like to dry up a lot. But I'm sure you won't have a problem with having to "blow the cobwebs" off of your car haha.

1983Zsled

Sonix...you are going to love your wideband...just remember to let her warm up good. I'm thinking it takes the engine up at operating temps before the sensor is really accurate.

I hear you on the rods....I think I have the fatter .043" just inside of the .063 jet. If i stomp on it from a dead stop it never goes over 13.8 :1 and once the 4 barrels open all the way you can watch the A/F change as she goes through the gears. The richest it is 11.8 :1 around 5000 rpm. Maybe A little rich for vortec heads IDK? I will try to dial it in more this summer.

The best way to describe the lean APT, is that there isn't enough of a decrease in vacuum to allow the rods to pull up off the APT stop at slight throttle while cruising, so the engine runs off the idle circuit and the little bit of fuel that gets between the rod and the jet, but stepping on the gas even a quarter throttle will slowly lower intake vacuum and allow the rods to move up in turn richening the mix. That's how I see it in my twisted brain anyway..lol

Sonix

iTrader: (1)

Holy cow, 63 down to 43 rod? that's a small jet right there. But hey, if your WOT on the primaries aren't lean, you're fine.

You're right on the APT, that's how I believe it works. The only thing is, if your APT is adjusted "right" (such that the rod is just barely on the .043" part in the jet when resting on the APT) then that's what you'll get at cruise (say 15" of vacuum or something) --- ie .063" - .043".

If your APT is adjusted to put the rod too deep (saw a guy do that one, tightened the APT until it bottomed out...) it'll bog on acceleration because the rod doesn't come up fast enough so it goes lean. It is running on idle mixture, or WOT mixture, there's no cruise mixture, so it's ultra lean. Actually, it could be on the .060" part of the rod (the main part, the part that is too thick to even be in the jet really...)

If your APT is too high, ie the rod is on the power slope during cruise, you can do that, but it'll come in rich quite quickly.

In all honesty, Q-jets haven't been made in 25yrs or so, so what we're doing with them may not be the way the engineer intended 50 years ago, but if it works for us.....

1983Zsled

Sonix...I made a typo, my bad it is .073 Jet...lol
I agree a great carb, but it took me all last year to really understand what I was doing with the mods I made. Glad to see that the way I understand APT is the same as you... I'm sure the 1983 Q Jet I had on the original LG4 305 was never primarily designed for a mild street 355. Interesting enough though, after all the playing around with rods and jets and secondaries, I ended up with the original configuration of .073 Jets with .043k rods. I did change the secondaries to CP from the stock CH rods. Seems to work good for me. I had read somewhere on the net to use the stock config as a baseline and work from that. I would of saved myself a crap load of time if I had followed that advice. Live and learn I guess !!!!

ANDYZ28

I know this is an old post, but I have this same problem. My 838 roller engine seems to be developing this problem. I now have about 800 miles on the engine and this issue has become one of safety in the morning when I pull out into traffic I must be very careful with the throttle or the car will just die right in speeding traffic. I hope someone can advise me on what to do. This carb was new in the box when I started this project.

Thanks, Andy

1983Zsled

Hi Andy, just to get some more info from you,
is your motor a 383 roller?
And is your carb a q jet?
Has it been rebuilt?
When do these symptoms occur?
Is it just first thing in the morning or is it all the time?
If it is a q jet has it been modified? If so what has been done, rods jets,etc.....
What cam do you have in your motor?
What your compression ratio?

I ask all these questions because the carb has to be tuned to the motor. Based on your description it could be any number of issues from choke set up, being too lean, too rich, or even a clogged fuel filter. I'm willing to try to point you in the right direction......

ANDYZ28

It was/is a new in the box Edelbrock 1904 carb w/o any mods.
The 383 was build by me the cam is the stock GM HOT cam TH700R4 with a B@M hole shot 2000 converter.

It occurs mostly at warm part throttle.

Thanks, Andy

1983Zsled

That carb is a large main air bleed carb with large jets and fat tipped primary rods. Looks like from the Summit Racing web site specs it is calibrated out of the box for truck engines built more for torque which would have a smaller cam than what you have. I would suspect that your carb causes a lean at idle and during acceleration the carb isn't rich enough for what your motor needs. You may be able to richen it up some on idle by adjusting your base idle first, then moving to the APT ( at part throttle) out a turn a time to see if it helps. If not you may want to also change the power piston spring. Go to cliffs performance.com and read some of the posts in the forums there. I'm no expert, but many of the guys there really know their stuff.

ANDYZ28

Thanks, I have ordered replacement springs from Summit. We'll see if that helps.

Thanks, Andy

ANDYZ28

I finally have the springs, but now what. There are 6 springs in the kit. Do I start by installing them in a trial and error method? Thanks, Andy

1983Zsled

Hi AndyZ28, The springs should be colored to correspond to a specific weight.
Pull the rods and spring out and compare what you have to the ones you received.
Look at the summit part number ( i couldn't find the correct spring for the step up in a Edel 1904 carb) and it should list the order of spring ie;
3 in. Hg blue springs
4 in. Hg yellow springs
5 in. Hg orange springs
6 in. Hg pink springs
7 in. Hg natural springs

If that doesnt help you to determine the weight of the springs you can always build a test stand for tour springs. I made one a while ago with a inverted nail that will support the spring. Place a washer on the end and then ad weight to see how much it takes to compress the spring. Obviously the more weight,= the stiffer the spring.

Remember the 1904 carb is a Quadrajet knock off so the normal step up springs for the 1405, 1406 etc don't apply. however you may be able to use one if they are the same size.

With your cam i would suspect off idle there isn't enough manifold vacuum to hold the rods in the jets, so what you need is a softer spring. If stock is 7 in. Hg, try 6 in. Hg and so on. It may not fix your issue but its worth a try. Do you have a manifold vacuum gauge? if so put it on a manifold vacuum source and see what it says at idle.

If your springs are no good, PM me and I will check my stock. I think I have a bunch of them in my collection all from Quadrajets mind you but I think them would work.

Good luck.

ANDYZ28

Well everything was going great until I disassembled the carb, then I opened the package containing the 5 replacement springs (from Carb specialties). I carefully positioned them on the front of the message from you, when I went inside to call them, the springs all blew away in the slight breeze including the original one. Damn it! I was able to locate one of the springs, which appears to be blue. But it was not the original one. Fortunately I did manage to get a good picture of them all lined up, the original one appears to be about 3/16" shorter than the rest?

Now I realize that the 1904 is an aftermarket carb, this might explain why the replacement springs are shorter. I do not know how to get the picture out of my camera, so I have uploaded them to my facebook page where you can view them "Andy Sherrer".

1983Zsled

Andy, tough luck on the wind blowing everything away! You can try a large magnet in the area, maybe you'll recover a few... I think you are starting to understand the Edelbrock Carb you have is a Quadrajet knock off. As stated I suspect the springs you ordered are for the 1405/1406 series not your carb that's why the length difference. I don't think that spring will work. Here is what I would do. look up http://cliffshighperformance.com/ and give them a call. the number is at the bottom of the page. They will have lots of questions about you r motor, cam, carb etc, but they should be able to sell you the right spring. These guys have a tonne of experience and have supplied me with many parts over the years. I have built 3 quadrajets using Cliff's book and carb parts supplied from them, every one of them was very close requiring only minor adjustments to get it dialed in.
They may even have some other suggestions to try first based on their knowledge of that carb.
Good Luck!

ANDYZ28

Not exactly sure about this, but I installed the weakest spring and it seems to now be working just fine now.