q-jet idle

[prostreet64]

Good morning one and all. I posted this question over on PY but no responses yet. Anyway, I have a q-jet that the rpm will hang at 1500 when I blip the throttle. I have the idle set at 900-1000 when I first rev the motor. I am then manually closing the throttle against the fast idle stop to make sure it is closed, but the idle hangs at 1500. If I back off the idle screw the idle will come down and then I can reset it at 900. If I repeat the test I get the same result. Fuel pressure is at 7-7.5 psi; could this be causing the needle to stay open? This carb was rebuilt several years ago  by an unknown party and then put on the shelf so it hasn't been run in a longtime. Other things to look for? Thanks.

[gtome]

Any chance the butterflys are hanging up on the intake? I had one act like that before and thats what it was doing. Mine had a 4 hole spacer on it just FYI.

[prostreet64]

I thought about that but I was manually closing the throttle linkage at the carb  to make sure nothing was hanging up on the spacer. Even before that I had looked down the throat and worked the butterflies full open to make sure that everything was moving freely.

[429bbf]

fwiw is there a chance that its hanging on the fast idle cam . that what it sounds like.dean

[motorvation2go]

Good points for investigation have been suggested.
Having eliminated(?) these as the source of problems, I can't help but think of vacuum leaks (hoses, carb to manifold, manifold to heads, over enthusiastic drilling of holes in primary throttle butterflys)...
*** Also gnawing at the back of my ol' noggin is the possibility that you're running [too] light centrifugal advance springs in your distributor (and/or sticking weights) which might be causing your timing to 'hang'.
I'm sure Cliff will have better insight into your situation... but the timing issue deserves consideration.
Good luck with the chase...

[Cliff Ruggles]

Verify that no part of the mechanical advance curve is being added, or staying in right off idle.  I see this a LOT, and it will cause the exact problems that you are having.....Cliff

[prostreet64]

Thanks much for the responses. I should have explained a little more in my first post. This q-jet was borrowed from a friend so I could keep my son's 69 Bird running for street use while the Demon 825 was sent out for a hi-po tuneup. Just wanted to get the q-jet running well enough for the weekly around town cruise; no track time planned. Anyway this is no longer an issue as we have decided to build a q-jet (not this particular one) for the Bird instead of working on the Demon. However, I would still like to figure out what is going on with this particular carb. I am aware of the potential for light springs to allow advance at or right off idle; we have dealt with that problem in the past, but believe me our combo is dialed-in and that is not the problem. We have the idle set at 1000-1100 rpm with the Demon carb. and there is no mechanical advance at that rpm. If we blip the throttle the rpm will settle right back down to the same range. There has to be some other issue that is causing the q-jet to hang at the higher rpms. I hope to have time on Thursday to put the carb back on the Bird and take a close look at all the linkage; maybe something will turn up.

[Cliff Ruggles]

1100-1200 rpms idle speed would have nearly any mechanical advance bringing in some timing unless it had pretty strong springs on it.  Even stock advance curves are already adding timing by then. 

As far as the carb goes, you are probably on the main system, as most likely the idle system isn't tuned exactly for your combination.  This will make it difficult to hold a stead idle speed.

It would help to know the engine specs, and exactly what's been done to the carb you are trying to use?.....Cliff

[prostreet64]

Cliff, I sent the engine specs a few days ago, via your web site, with an explanation that we would be looking to build a q-jet for the Bird instead of fooling with the Demon. And you gave some recommendations as to the best cores to look for. I know you are very busy and probably didn't make the connection between that e-mail and this current post. Anyway, what I might try is pulling the q-jet off the TransAm and putting it on the Bird just to see what happens as far as the idle is concerned. With the Demon carb we found it idled best at 1000-1100 as mentioned before; could be some  advance at that rpm although we do run fairly tight springs. I am hopeful that a well built quadrajet will offer a smoother and lower idle; even the one we have been discussing idled better than the Demon at 850-900 rpm.
Gates

[Cliff Ruggles]

Typical idle speed for auto trans cars is around 650-750 rpm's.  Manual trans cars with aggressive cams are usually happy around 800-900rpms.  There is seldom any benefit idling over 1000rpm's with either type of trans, and most auto trans cars "hit" pretty hard when placed in gear, if the engine has a high idle speed.

I've seen combinations a that require a pretty high rpm setting out of gear just to stay running.  Most had pretty big cams in them.  If the cam is well chosen for the application, even a pretty big cam can be "tamed" with correct carb and distributor tuning to get good idle quality at "normal" idle rpm's.....Cliff

[prostreet64]

Cliff, that is exactly what we are looking to accomplish by going to a q-jet. It might be possible to achieve a 'good' idle with the current Demon carb. but we want to give the q-jet a try, and besides I always like to see the look on peoples faces when you pop the hood and they see you are running a quadrajet instead of a Holley or equivalent. Our cam in the Bird is on the 'big' side for a 400 motor; 248/254 @.050 but even so I am sure we can improve idle quality with the right carb and timing setup.

[Cliff Ruggles]

That is a big cam for a 400, hopefully it's a high compression engine?  Still no problem getting it to idle well.

The q-jet has an excellent idle bypass air system which really helps idle quality with big cams. It diverts idle air around the throttle plates and comes in under them.  This is a controlled vacuum leak, and big cammed engines need this to help keep the throttle plates low enough to stay on the idle system instead of pulling fuel from the main system.....Cliff

[prostreet64]

Cliff, the c.r. is right at 9.7. Over the years as we have looked for more horsepower, our goal has always been to maintain a pump gas friendly combo, so with the iron heads we are keeping the c.r. in a safe range. I was talking to my son last night and we are pretty excited about getting a q-jet and rebuilding it per your recommendations and specs. As soon as we have a core I will be ordering the parts that we need.
Gates

[prostreet64]

Cliff, my son was talking to a friend of his that has a repair shop and after he learned that we planned to replace the Demon with a q-jet he gave us 4 units that he said had been sitting around for a long time. They all appear to be Chevy applications; one 1971, one 1972, one 1980, and one 1984. Here are the #s, do you think one is a good candidate for our buildup? 7042208, 7041209, 17080204, 17084226. If one of these is a suitable core then I am ready to get the parts on order. Thanks,
Gates

[Cliff Ruggles]

I'd use one of the later ones, preferable the one that is in the best shape and most complete....Cliff