Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

Rich off idle stumble.

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novadude:
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!).

77cruiser:
Nice, DMAB leans out at higher RPM/flow just like Tuner said.

novadude:

--- Quote from: 77cruiser on August 11, 2025, 02:54:58 PM ---Nice, DMAB leans out at higher RPM/flow just like Tuner said.

--- End quote ---

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.

77cruiser:
How much did this one change your MPG?

novadude:
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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