Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

Idle problem

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dlphil10:
Background:
I have recently rebuilt a 350 Pontiac motor. The specs are as follows:

350 with 0.030" Overbore
061 heads which are supposedly 72cc heads but published values are known to be a little off
Stainless steel valves: 2.11" Intake 1.77" Exhaust
Crower 60420 Cam: 270/276, 210/221 @ 0.050" lift, 0.421"/0.446" lift, 112LSA
Automatic Car (TH400)

Carburetor: 7029262 which from a 428 motor
The carburetor was rebuild using cliffs kit that had the most components and the throttle shafts were bushed using components from this site.
I believe the primary jets are 72s and the metering rods are 46s

I have just broke in the cam with 40 minutes total run time at 2500 rpm +/- 500 rpm. Brad penn break in oil was used and only the outer valve springs were used to aid in cam break in.

Problem:
Whenever I was breaking in the cam the car had a very slight stumble but it didnt seem too regular. I was told to proceed with breaking in the cam, then tune the car afterwards. When I went to tune the setup, the idle was a bit rough(in park). The timing was set around 10-12* and the idle was ~550rpm-600rpm. I got the vacuum at idle to be around 15inHg by backing out the idle screws, but now I think that my expectations of a higher idle vacuum were incorrect and misguided me into possibly leaning out the idle. Whenever I was breaking in the cam I did notice I had around 20-21inHg of vacuum strong and steady.

I dont think my vacuum advance is engaging at idle, but could that cause this issue (there is that slight stumble at high vacuum cruise scenario). I ordered a crower adjustable unit just in case.

Could an incorrect power piston spring cause this problem? (I was expecting a lot more vacuum at idle and chose the spring that seemed most like the stock spring in length and tension)

I know thats a lot of info, so let me know if you have other questions.

Thanks!

dlphil10:
Also note that the spark plugs are slightly tan, not as tan as I have normally seen, but I attributed that to the cam break in with little load. Is that an incorrect assumption?

dlphil10:
It should also be noted this casting has idle bypass. I couldn't imagine that the build would use the same air as a 428 car, so I set the primary throttles so the slots aren't exposed at idle.

Cliff Ruggles:
Pretty lean jet/rod combination for what you are doing.  For sure I would install smaller primary metering rods to add some fuel to the part throttle part of the curve.  The idle system should be adequate for that engine combination.  Make sure to remove the triangular metal cover over the vent tube, there is a tiny screw under that cover that control idle air to the upper bleeds, which can be used to fine tune the idle system as well.....Cliff

dlphil10:
Thanks Cliff! What would you suggest for the primaries? Should I go down to about a 42?

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