General Category > Quadrajet Carb Talk and Tips

q-jet idle

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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

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