Rich off idle stumble.

[77cruiser]

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.

[Kenth]

18 deg initial is a lot and to me it indicates a too lean idle circuit.

What carb and recipe?

[77cruiser]

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

[novadude]

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?

[77cruiser]

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.

[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]

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.

[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.

[77cruiser]

MPG increase is always good unless you lose performance. Good job John.

[Cliff Ruggles]

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.......