Cliff's Quadrajet Parts and Rebuild Kits
Quadrajet Problem Solving => Diagnose a Quadrajet carburetor problem => Topic started by: 77cruiser on August 09, 2025, 02:21:35 PM
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I'm getting a stumble off idle light throttle tip in. I have the linkage in the outer hole, was worse on the inner hole.
Combo 421 SBC Vic JR. 227/239 @ 0.05 hyd roller 9.8 CR 18 deg initial w/10 deg manifold vac.
Pretty sure the accel discharge hasn't been opened up, I didn't note it on my sheet.
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18 deg initial is a lot and to me it indicates a too lean idle circuit.
What carb and recipe?
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17084226
Idle tube .031
Idle down .052
Idle disc. .093
IABP .136
Idle AB moved to air horn .043
Lower idle bleed .062
MAB blocked drilled for Dual MAB .046
P jet .074
P rod 52M
Inlet seat .144
Float 5/16
SAB .026
Hanger G
Rods CV
Air valve 1.29
Goes rich at light tip in to about 10 AFR then back to 13-13.5
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Your idle calibration is similar to mine... I have .029" idle tubes, .043 UIAB in air horn, .062 LIAB, .046 IDCR. Using .046" Dual MABs, and 74 jet w/ 54M rods. Idle air bypass on mine is 0.089". I have less cam (217/225 @ 0.050, 108 LSA). I have 13 deg initial with ported vacuum advance (15 deg can).
I wonder if reducing the IDCR would help with the rich spike?
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When did you build that carb? How's it working out compared to the other one?
I took it apart & found one pull over tube on the sec. was off, brake booster fitting was loose. Nothing that would make it rich though, but I was able to turn the idle mixture screws in about a full turn & it's better. I might try putting a IDRC or put a bigger IAB in it.
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I built this one ~2-3 years ago. It's based on a 17080204. This is a small primary (750 cfm) DMAB carb. It is working very well. If anything, it is just a little lean at off idle and ~1500-2000 rpm part throttle.
I dropped down to .040 UIAB and raised the APT a full turn this weekend to see what impact that would have. I've not driven it yet. A/F meter shows 14.5-16.5 AFR under all part throttle conditions from off idle to 80 mph cruise.
Riding around at ~1500-2000 rpm, it is ~15-15.5:1, and I think that may be a bit lean. If you are really paying attention, you can feel an occasional "skip" or miss. Barely perceptible, but I am going to see if I can tune it out.
70 mph cruise on the highway is ~15.5-15.7 AFR, and it recently got ~20 MPG at a 3200 rpm cruise speed on a recent road trip (small, light Chevy II helps for sure!).
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Nice, DMAB leans out at higher RPM/flow just like Tuner said.
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Nice, DMAB leans out at higher RPM/flow just like Tuner said.
Yes... I have noticed this. Compared to my previous SMAB carb, this one seems to behave more "textbook". Seemed like the 0.050" SMAB would hold AFR flat, or even trend a little richer when pulling hills on the highway, etc. The DMAB trends leaner until vacuum drops enough to let the power system start to come in.
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How much did this one change your MPG?
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At highway cruise, not a huge difference... 1 mpg maybe? The SMAB had a similar AFR at 70 mph cruise speed, so when cruising on level ground, there isn't much difference. My perception is that it is a little better around town, but I've never measured it.
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MPG increase is always good unless you lose performance. Good job John.
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Idle AB moved to air horn .043
Lower idle bleed .062
That's going to put a LOT of fuel to the mixture screws and transfer slots. I'd start by opening up the LIAB to .067" and see if that helps.......