Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor
Poor Fuel Mileage on 4.3 V6 with Quadrajet 17056250
don3194:
I installed the bushings in the original baseplate and reinstalled the carburetor with the original baseplate. It still seems to be leaking air from somewhere, its a lot better though. I actually have the idle just around where I want it, I'd probably want it about 50 rpms lower but the idle screw is backed all the way out. I can live with that but still makes me wonder what I'm missing.
I haven't rechecked timing, but couldn't too much initial advance artificially raise the idle?
Kenth:
Check for too short throttle wire/rod holding the throttle blades open some.
Cliff Ruggles:
Yes, if the initial timing is pretty high on a "stock" or very mild application you can loose control of the idle speed.
EVERYONE doing this sort of thing will Google up the subject and switch or use manifold vacuum to the advance putting the initial timing WAY off the scale or at least 25-30 degrees or so. That's all fine if it works for you but some engines make a LOT of vacuum without much timing in them and adding a bunch more will have them idling too high even with the speed screw backed clear out.......Cliff
bry593:
--- Quote from: don3194 on March 09, 2021, 09:50:33 PM ---I'd probably want it about 50 rpms lower but the idle screw is backed all the way out. I can live with that but still makes me wonder what I'm missing.
--- End quote ---
To further lower your idle, I recommend plugging your idle bypass air (IBPA). Just drill and tap the throttle plate and install some 1/4-20 setscrews. There are pics of this mod on the internet. This trick allows using carbs from larger displacement 350 engines on smaller motors like your 4.3 V6 or a 305 V8.
Once IBPA is plugged, you will have control of your idle screw. This will allow you to open the thottle blades just enough that these are not sticking in the primary bores. I'd bet with the idle screw backed all the way out, you currently have a bit of stick/slip when first applying throttle.
By the way, the IBPA is probably your mysterious vacuum leak. That's what it is, a controlled vac leak used by Rochester to raise or lower the idle so that the idle screw works within a small window. The range of usable idle screw is from blades fully closed to thottle angle which begins to draw fuel from the transition slots. So, fairly narrow range, hence the IBPA.
don3194:
Okay, I verified timing, verified my throttle cable isn’t restricting. The throttle plates are all completely closed. I blocked the idle bypass port you suggested to block bry, and that made 0 change whatsoever. I’m baffled that it didn’t change anything. The idle will not go below 800 rpms, I can live with it being 50 higher than where I’d like it but I mean it shouldnt run with the blades sealed.
One thing still to note is that I can hear a nice whoosh/sucking sound through the primaries at idle.
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