Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor

Please help me dial in this Q-Jet

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77cruiser:
Try the IAB. Going back & looking the idle tubes being that big is a major factor, if you have a way to get them down to about .033-.035 it would help. Carb must have been modded for a more radical cam at some point.

67RedC10:
I believe the carb came off a 1977 GM truck that had a 454, but it's possible too that it was modified. 

67RedC10:
Okay, I've tried various changes and I'm finally about there.  I pulled the IAB plugs and installed different plugs with holes drilled in them. I first tried one with a .077" hole and the other with a .107" hole and that helped quite a bit except it increased my idle a bit too much and the throttle plates won't close any tighter.  I was still a bit rich at really light throttle so I tried drilling my upper idle air bleeds from .075" to .080" and that helped also.  I then drilled my lower air bleeds from .063" to .068" and I was getting really close.  I then drilled the lower air bleeds to .072" and that was pretty much the ticket.  I also ended up very lightly stretching the power piston spring a little (probably a sin to do that) and that cured my off idle lean stumble.  I tried a next smaller primary jet change and that was a wrong move so back in went the .066"' ones. 

The last move I'm planning to make is change the .107" IAB plug to a .077" like the other one so I can drop the idle speed 50 rpm or so.  My AF ratios are now really good at all throttle areas and loads and it's driving very nice.  We'll see how much better it is this winter compared to last! 

Cliff Ruggles:
I'm slightly confused.  What are IAB plugs?

Use the APT system to correct an off idle lean condition by raising the .042" metering rods a little higher.  Stretching out the PP spring is the WRONG way to tune light throttle "tip-in".  The power piston should be down at idle, and when accelerating lightly from idle to the main system.  It's sole purpose is to richen things up and heavy throttle openings, or when vacuum falls low enough to call for fuel enrichment.  At the same time the vacuum advance timing falls out, to keep from pinging the engine.

So tune all light throttle situations with jets/metering rods/APT system.

The idle tubes are HUGE for what you are doing, with .075"/.063 IAB's, it would have been fine with much smaller idle tubes.  A slight amount of bypass air would most like help it as well, based on the comment in one response about removing a vacuum hose.

That engine should not want or need a lot of timing at idle speed either, and be fine without MVA.  I've tuned a LOT of those engines over the years, and haven't found one yet that needed any additional timing at idle beyond the base setting......Cliff

67RedC10:
I had plugged the idle bypass air by installing 1/4-20 set screws in the idle bypass air passages that go through the base plate.  I tapped the holes on the top side of the plate and screwed in the set screw "plugs".  That was the only way I could get my idle to drop down.  It used to be really bad until I installed new bushings for the primary throttle shaft.  Even still though with the new bushings and my throttle plates fitting real nice with no daylight showing around the edges when closed tight I still had a high idle speed.  With those "plugs" installed my idle came right down perfect with the throttle plates just barely cracked open.

It sounds like what I really needed to do was to pull the idle tubes and install replacements that were quite a bit smaller., but I must say its running really nice at the moment.  Those idle bypass air "plugs" in there now are really short pieces of 1/4-20 bolts (without heads) that have small holes drilled through them and the top slotted for a screwdriver to install them.  I need to replace the one that has the .107" hole with the new one I have that has a .077" hole the same as the other one.  That should bring my idle back down an additional 50 or so rpm that I want.

If I have a lean off idle condition I'll raise my power piston with the APT screw.  This thing has always run best with the timing set at factory specs but using manifold vacuum advance.  Possibly with the advance curve altered and the total amount of vacuum advance limited ported would be the better option.  Cliff I wish I lived close to you and you could help me perfect the tune on this thing.  I know I'd learn a ton and have a ball doing it.  How are you with Holleys?  I need to tackle my old truck next.

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