Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor

Please help me dial in this Q-Jet

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67RedC10:
Hello everyone,

I have a Q-Jet p/n 17057229 (from a 1977 truck I believe) that is running rich at really light throttle.  The engine is a pretty much bone stock 1971 L48 270hp 350 from a '71 Chevelle (points and all) but is installed in a Toyota Land Cruiser with a 5-speed manual tranny.  The engine has about 8.5 to 1 compression and has cast iron ram horn exhaust manifolds with dual exhaust.  I have the mixture screws out 1.5 turns each and I'm right at 13:1 AF Ratio on my LM1 meter (at 650 rpm).  That's where I get my best vacuum reading (18 inches) and if I lean it out any from there it begins to pop a bit out of the exhaust.  13:1 is almost dead smooth. 

If I rev the engine in neutral from 1000 rpm to just below 2500 rpm the AF Ratio is around 12.5:1, then as soon as it get above 2500 is begins to lean out.  When I drive down a flat road under light throttle (such as 4th gear at 2000 rpm) my AF Ratio is rich at around 12.5:1 (the throttle is just barely cracked open).  If I maintain the same speed (2000 rpm) as I go up and over an interstate overpass my AF Ration goes to about 14.5:1. If I increase my speed on flat ground to 2300 rpm up through 3000 rpm and beyond my AF Ratio is around 14.5:1.  If I get into the throttle heavy without getting into the Secondary's my AF Ration drops to about 12.8:1.

I have the timing set at the factory specs (8 degree initial advance and stock spec vacuum advance canister connected to manifold vacuum).  I've tried advancing and retarding the ignition and it's happiest at 8 degrees initial.  I also tried ported vacuum advance and it prefers manifold.  I tried leaning out the idle mixture to 14.5:1 and increasing the ignition advance, retarding the advance and trying manifold or ported advance.  It prefers manifold advance and 8 degrees initial along with AF Ratio of 13:1.  If I disconnect the vacuum advance from manifold vacuum and plug the manifold vacuum port that the hose came off of the engine slows down and doesn't run as smooth.  If I unplug the vacuum advance from manifold vacuum and leave the vacuum port open to allow some additional air into the engine the engine actually idles quite nice and the AF Ratio registers about 14.5:1 on the meter.  But if I adjust the mixture screws to give me 14.5:1 idle AF Ratio then disconnect the vacuum advance and plug the port the engine doesn't run as well and pops a bit out the exhaust.

Okay, where should I go from here?  I'm thinking it's just getting too much idle fuel and maybe I should enlarge the Upper Idle Air Bleeds a bit?  Or should I decrease the Primary Jet size a bit?  My AF Ratio with heavy throttle (without Secondary's) I believe is very good so I hate to mess with the Primary Jets and mess that up.  Thanks for any suggestions.

Prior to all this I rebuilt the carb using a rebuild kit from Cliff and here's the specs after careful measurements:

Idle Tubes - .038"
Idle Down Channel Restrictions - .055"
Upper Idle Air Bleeds - .075"
Lower Idle Air Bleeds - .063"
Idle Bypass Air - .139" (currently plugged)
Mixture Screw Holes - Drilled to .086" (originally .060")
Main Primary Jets - .066" (originally .063")
Primary Rods - .026" and .042" (originally .026" and .039")
Secondary Rods - DG - .034" and .134"
Air Horn Primary Air Bleeds - .052"  (Single Main Air Bleed Carb)
Inlet Fuel Seat Diameter - .130"
Acc. Pump Discharge Holes - .026"
New Power Piston Spring installed that was recommended by Cliff.  I didn't write down the specs.
Installed Cliffs APT Screw in the Base Plate.  I haven't fooled with this much yet.  It's set to just start raising the rods.
I didn't include the Secondary Specs since that is working well.

77cruiser:
If you give it some IAB I think it will lean out some. The IAB is just a controlled vacuum leak.

67RedC10:
Do you recommend enlarging the upper or lower idle air bleeds (IAB)?

77cruiser:
I'd leave the air bleeds as they are, & put the Idle Air Bypass IAB back to function.
How is it blocked off now? Maybe it could be opened up to about .05-.065.

67RedC10:
Yeah, that sounds reasonable.  I drilled and tapped the top side of the base plate where the holes were and installed small set screws.  I could drill a small hole in each one.  I originally plugged them off because I couldn't get my idle speed down low enough.  I installed new primary shaft bushings and that helped a lot but still couldn't get the idle down below about 850 rpm.  With those plugged I now have a 650 rpm idle and my transfer slots are just barely exposed (like perfect square holes when against the throttle stop screw).

What would happen if I drill the lower idle air bleeds a tiny bit bigger?  How does that compare to drilling the upper idle air bleeds a tiny bit bigger?  But first, I'd rather drill those set screws since those are easy to change.  I have a small drawer full of those. :)

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