Tried to adjust my QJet and ran into some problems. I was adjusting the idle mixture screws and noticed that while adjusting them they had no affect on the motor. I turned them all the way in and no change, turned them almost all the way out and still no change. Isn't the motor suppose to die if you turn them all the way in? Every other car I have adjusted the idle mixture srewsin made a difference in the way it ran. Can anyone tell me what kind of response I am supposed to get while adjusting them?Is it different in a corvette? What am I missing? The engine has 90K on it and I don't think the carb has ever been rebuilt.Need advise

Hi L82Steve, I'd start by looking for vacuum leaks around the carb and manifold. PG

Thanks for the reply. I've checked the carb and manifold but found nothing. Is there a certain way to check for vacuum leaks? If I have a vacuum leakwould I be able to close the idle mixture screws all the way and keep the car running? When I drive the car it cuts out like it is starving are these all symptoms of a vacuum leak?

L82 Steve.

Hi L82Steve, it sounds like there are other issues with the carb, with the idle mixture screws all the way in it should have stalled out from too lean a mixture, the power piston may be stuck in the up position which would give you a rich mixture all the time but I don't think it would be enough to keep the engine running with the screws all the way in. You don't know the history of the carb, someone may have made some improper mods or adjustments that may be causing the trouble. I would tear the carb down completely and check for damage, if it looks good then rebuild it with a kit along with a new float and choke pull-off. Let us know what you find.

78Buckshot,

Thanks for the reply. I don't know anything about the carb but at least this gives mea direction to go in. I'll check it out and let you know.

Thanks again

L82Steve

Get yourself a copy of HP Books Rochester Quadrajets. Has a lot of good info in it. Also the factory service manual has all of the settings. Sounds like a good rebuild is in order. You can check for vacuum leaks with an unlit propane torch. Just point the nozzle around the various carb and intake gaskets and hoses and listen for a change in engine speed.

ORIGINAL: L82Steve
If I have a vacuum leakwould I be able to close the idle mixture screws all the way and keep the car running? L82 Steve.


No.

It doesn't matter if you have a vacuum leak or not. If all else is in order, and you seat the mixture screws (all the way in), it's gonna' shut down.

78 was right. And the acceleration pump might be stuck in the 'deliver'position.

Seems also, if the acceleration pump can't deliver enough fuel to keep the twister turnin', even with the mix screws seated, then the only other place the fuelis gonna' come from is the secondaries, and even then, only if they're vacuum secondaries, as opposed to mechanical.

what rpm is it idling at,if it can idle.check rear carb butterflies and make sure they are closed.is it dripping in the carb when its idling.check curb idle screw isnt open too much,bypassing idle circuit.are there mixture limiters on the 2 screws

It idles at 900 with a slight miss. I just recently rebuilt the distributerand it runs much better then it did. I noticed that it had a slight miss so I thought I would adjust the carb and found that it doesn't adjust. When I drive the car it cuts out like it is starving for fuel when I step on it, as soon as I let off it idles fine. I have timed it to spec. It has anew fuel filter/pump,plugs,wiresand fuel line. Thanks for all the great advice. I have a few things to look at and I'll get back to you and if you can think of anything else please let me know.

Thanks again

L82Steve

Check your plug wires for firin' order?

Of course that doesn't explain the mix screws non-function tho'... That needs to be addressed irregardless.

900 is just a tad much,try lower if it will stay running and adjust[after ya get rid of miss] on cut out : engine off look in carb while opening linkage you should see 2 streams of gas being shot into intake like 2 water pistols from front barrels.q jays are famous for acc. pump failure.on miss : loosen wires on dist cap,with engine running grasp wire an inch from boot and pull from cap you should hear/feel a miss,reinstall then do another cyl. the one that makes no diff. is the missing cyl

Hi L82Steve, can we backtrack a little? I see your in California, is the car originally a California car, is it an auto trans, stock engine, ignition, carb., etc. If we can get more detail on the equipment we have a better chance of nailing down the problem(s). Extanker is heading in the right direction, the engine is idling too fast at 900 RPM, check the specs. sticker under the hood for idle, timing, etc., I think it should idle around 650-750 in drive if it's an automatic. When you adjusted the timing did you unhook and plug the vacuum hose from the distributor first? Also since I'm not very familiar with the Calf. emissions equipment can you tell us if the Q-Jet is an electronic controlled carb.. if so it will have a two wire plug at the right front top of the carb. Give us as much precise info as you can describe.

ORIGINAL: extanker

900 is just a tad much,

Yup. But his problem is not fuel delivery failure; it's OVER-delivery.

But don't even worry about that right now. It's gonna drop when the mix screws allow a little more fuel in, like they're supposed to do.

http://home.comcast.net/~whitehause/bad_kitty.jpg Muhahahaa....another Q-jet in my sights....must rid the world of Q-Jets. If I had a nickle for every problem I'v had with my Q-jet I could afford a Holley! Seriously...unless your dead set on keeping it stock(assuming the Q-Jet is a stock one)buy a holley. Then........go to your back yard....build a stack of logs 4 feet high.....set the Q-Jet on top.....light the logs.... bask in the only warm feeling a Q-Jet can bring.

Sounds like you need to rebuild. Ports for mixture screws could be varnished up and therefore won't make a difference where you have them. A rebuild kit and cleaning and blowing out all the ports should do the job. Carbs will get that way if you don't refresh them once in awhile. When was the last time you went through it?

I agree with Texas. :)
Beleive me, it's not just Q-jets. ( Roachnesters :D)
All brands will do it. ;)

( Ha...Ha....Hollys :D)
( But Carter's got the pills for what ails ya. :D)

Looks like we're just holdin' this discussion between the folks that wanted to help, haven't seen L82steve give us anything else to go on.

This thread has had over 150 hits, so I bet cha somebody, somewhere, is learnin' sumpin'. ;)

Good point Dave, I new there was a reason you are a moderator and one of my hero's. If anyone has something to add or questions about Q-Jets we can continue on from here.

I just got rid of a '86 chevy p/u that I put an old AFV carb on (mid '60s 4bbl that chevy used) and one of the mixture screws made just a small difference, the other did nothing. I hadn't gone through it since 2000 summer. The guy loves the truck. LOL I tried telling him the little things about it as it's a work truck, but it was like talking Greek to him (he's definitely not Greek LOL) and he didn't even react. So I just shut up.

I have some Carter AFBs, and have always had good preformance.
Ya, ya gotta "kit" them once in a while, but you have to do the others too. :)

Yes, the AFV is the older version of the AFB and the same basic set up. The AFV I had was the manual choke model and I'm not sure that they came with an automatic one. And I agree that they do perform well and easy to work on. For a long time, 12 years back and more,I used the Carter carb (750s mostly)and a Torker II or Victor Jr. on all the small block Chevys (and others) I built. That combo was just a favorite of mine for awhile. They (Carter carbs) ran good, were reliable and required little maintenance. I'll run a "Demon" on the next performance engine I build.

If you are idling at 900 then your main throttle plates are not closed and they need to be right closed to put the carb into its idle circuit. You need to turn the idle down and make sure the throttle plates are fully closed then you can adjust the mixture screws.If it is not in the idle circuit then turning the mixture screws will do nothing. Are you using a vacum guage to set the mixture or a tach?

Has this guy done anything with the carb yet? I suppose he may want to go through all the basic tune-up procedures including the carb. You're right, 900rpm is way too high for idling. And it very well may be that he's not using the "idle circuit." Some things are easy, and some things are tough unless you're "there" with afew tools to try a couple of things.

Sorry I haven't given an update sooner but I haven't had timework on it , anyway I finally pulled the carb, took it apart and found that thepower piston was stuck open. The bowl had a layer of varnish covering the entire bottomabout 1/16 thick and the jets were filled with gunk it wasin bad shape, I'm surprise it even ran. I bought a rebuild kit, a book and did a lot of research before I tore into it and finally completed the rebuild about an hour ago. Everything went smoothly and I am looking forward to getting it back on the motor to see if my first ever rebuild was worth it. I appreciate all the great advice I will keep you posted on my progress andI'll try to get back to the forum sooner this time.

L82 Steve

Really glad to hear that the rebuild went smooth. My first rebuild was a one bbl. from my '65 Volkswagen bug in high school. You jumped in with both feet taking on the quadrajet your first go- round. And I see more than one person was right.
Let me tell you a good one; I've had my 'Vette for about 3 years now, bought it "all of a sudden" never having planned on buying it. It's never driven far, is all original and is a beautiful looking car. But this car needs to be gone through; ie: the suspension. Well, the carb on my 'Vettestill hasthe caps on the mixture screws. The carb has never been gone through. LOL It runs well, idles very smooth at 600 rpm and the accel. pump is working. LOL Do you think it's time?
Anyway, I am very glad that things turned out well and the best of luck getting it back on. The hard part is over and nothing but fun now as I'm sure it'll start up easy for you and run well.
I find with anything, that fear comes from "not knowing" and with a good manual and this site, all is possible for our 'Vettes. Thanks for the up-date.

I installed the carb today and thank god I labled all the hoses so I knew right where they went. It took a little to get it pumping gas to the carb but once it got up there it ran pretty good. I checked the timing again and adjusted the air fuel screws and they actually killed the car when I turned them all the way in so I know I did something right on the rebuild. I don't have a vacuum gage so I tuned them by ear. I just went on a 20 minute test drive and it ran great, smoother then it ever has. One question, I checked the sticker under the hood and it says that the idle should be set at 900 in park and 750 in gear. If I set the idle at 750 or lower in park it won't stay running when I put it in gear, does that sound right? Another thing I noticed is that the headlights come up faster since I timed and rebuilt the carb. How do I adjust the air fuel screws with a vacuum gauge?

Hey Jim, on your vette if it isn't broke don't fixit. I didn't have choice this time it was either fix it or don't ever drive it and I love driving it too much to let it sit. I just want everyone to know that I'am very thankful that there are so many people willing to help. Thanks to everyone who responded.

L82Steve

L82, I have to say that you jumped in with both feet taking on the quadrajet for your first carb rebuild. My first was a one bbl. on my '65 VW bug when I was in high school, that I eventually replaced with a two bbl. along with a dual point Mallory distributor. A screamin' 1600 motor and a "super beetle" crusher. LOL
I don't know if the engine should die at that high of an rpm whenyou run the idle down.(although it doesn't sound right to me.) That darn ECM controls the carb in alot of ways which is honestly something that I'm not totally up on. My engine stays running after backing the idle screw out to the point of being in-op, but that may very well be due to the mixture screws being so far out from never removing the caps to adjust them. (one of the reasons for me needing to rebuild my carb.)With anall-mechanical carb, I have not a bit of trouble, but with that ECM helping out, I have to go back to the GM shop manual to read and re-read different parts. The head lights being faster since the rebuild tells me that you fixed a vacuum leak in the process. Either that or the resetting of the timing bringing up the idle and inturn the vacuum level. Sometimes the leak occurs at the base of the carb due to the bolts coming loose over time from vibration. All in all, it sounds like your rebuild was a success. Thanks so much for the up-date. And again, a heck of a 1st. time rebuild!!!

ORIGINAL: L82Steve

If I set the idle at 750 or lower in park it won't stay running when I put it in gear, does that sound right? Another thing I noticed is that the headlights come up faster since I timed and rebuilt the carb. How do I adjust the air fuel screws with a vacuum gauge?

L82Steve


You might get away with 800-850 rpm in Park but probably no lower.

With the vacuum gauge connected to a manifold source (not a ported source), set the idle speed and then adjust the idle mixture screws for max vacuum. Will probably need to reset the idle speed while you do this. Also want to do in Drive for best results. Also make sure timing is correct before you start.