Where are the Q-jet guys? SMAB carb question

[novadude]

I've seen several talking about converting SMAB carbs to the traditional DMAB set-up. For those of you that have done this modification on a well-sorted SMAB carb, have you seen any noticeable improvement in part throttle A/F behavior? Is it worth doing? I'm mainly looking for more stable A/F at cruise, as my SMAB carb works great but tends to trend rich with RPM at light loads.

[77cruiser]

They haven't made the journey yet.

[TheTestPilot]

Hi
All the Qjets ,I have tuned are DMAB ,picked these at swap meets here in Oz ,
all from GM Holden V8's.
I can say these carbs go leaner til the power piston is released .
I know this doesn't answer your question ,but something to add
to the Qjet file.

[Tuner]

[novadude]

Tuner: Thanks for your input. To answer some of your questions:

Idle circuit:

Lower IAB: 0.059"
Upper IAB: 0.067"
Idle DCR: 0.052"
IFR (tube): 0.035"
Idle bypass air: 0.072"

This is pretty much "as delivered" for this particular SMAB carb, though I did reduce the Idle air bypass a bit.

When I say "trends rich", here is a better description. Not necessarily constant load... level road surface, high gear. Runs good with A/F at 16:1 @ 50 mph (arbitrary example), but will trend to 14.7:1 @ 80 mph. Load obviously increases with speed, but level road is still light load, and not enough to raise to the power piston and get into power enrichment. I haven't actually had a vacuum gauge on it, but I am assuming based on pedal position that vacuum is not dropping a bunch under these conditions, and I don't have a heavy power piston spring.

I am pondering how to flatten that part of the load vs A/F curve.

[Tuner]

What size is the single main bleed above the nozzle? What is the difference in intake vacuum at 50 and 80 MPH.

[novadude]

[77cruiser]

Yer gonna have to build up one of those Hybrids like I did & try.

[novadude]

[77cruiser]

Might have gotten a bit better MPG. I relocated the upper idle bleed too, so I think the light 30-40 mph cruise is a bit better.