IAB and it’s effects on transition

[440Duster]

I don’t want to start another argument/heated discussion but if someone wanted to extend their transition further out to help a lean spot as the mains start, can that be done by the IAB? My understanding is that transition can be extended to a point by smaller IAB’s.

[GTO Geoff]

You are correct.

[Right hand drive]

I’m with Geoff on this one.

[GTO Geoff]

Here is another thought, just thinking out loud.

Where are the pri blades at idle relative to the T slot? If more than 0.040" showing, you could try some bypass air. Easy to do if you a carb with adjustable BA. If not, you would have drill the t/blades or find another way to let more air in.
By using BA, blades are closed more, more of the T slot is available for transition. At the same point in throttle angle that the two systems merge, now with BA, the engine rpm will be slightly higher. That will create a little more depression & the main system would start earlier, idle system hang on longer. Might be enough to cover the lean spot.

[440Duster]

Thanks for the info folks.

Let me throw out a scenario that I'm chewing on. So this car is running pretty good. Still may be a little fat just off idle but not too fat to cause drivability issues. Where its bugging me is at the transition between the end of the transition slots and the beginning of the mains. The larger IAB did help minutely but not significantly. For reference I'm talking the cruise speeds between 50 and 60 mph in the rpm range of 1700-2000 where the AFR peaks at 17 +/- .2 until the mains come in and drop AFR's down into the hi 14's. It's not so bad to cause a major drivability issue but you can feel and hear that she's just not real happy about it (if that makes sense). Now to throw a curve ball. This car has 2 750dp's sideways mounted on a tunnel ram. It's also gasser styled with a straight axle that puts the carbs at a 4.6* angle. That results in one side of the bowl having a higher fuel level than the other. Which would mean that there is different fuel levels at each side of the metering block and with my limited understanding of emulsion would make the pressure differentials different on each stack. Theoretically that could cause one main to start later than the other, or, the transition to end earlier, correct? And if that's the case, it could be a cause of the lean spot. So I'm thinking to test that I could raise the float level and retest to see the effects. Another thought is to experiment with the emulsion stack, moving the jets up or down on one side to put them equally above/below fuel level. Am I making any sense at all?

The emulsion stacks as they are now from the top down:
.028
blocked
.028
blocked

Hoping to open up a discussion that not only clean up my cruise but further my understanding.

[GTO Geoff]

I thought you had a lean spot at the end of transition. Now you are saying it is fat?

[440Duster]

[GTO Geoff]

Don't get cranky with me you idiot.

You said 'to extend their transition' to help a lean spot...
I said 'A lean spot at the end of transition'.

The wording might be slightly different but the meaning is the same.

People have taken time to offer you some help, appreciate THAT.

[Right hand drive]

[rgalajda]

"Update on the Jalopy for those interested . Fired the thing up last night and adjusted the idle mixture maybe 1/16 turn in on all 8 corners. Leaned out a bit and got a bit more vacuum at idle also. So I decided to take it to work today and see what that did to the overall low speed cruise. Much much better. I’ll know better on the drive home, but I’m very optimistic. Seems that it may be a smidge lean at the transition from slots to mains but again I’ll know more later."

Is this an update to the above ?

That intake has a good 3/4 inch offset and an open plenum below the carb mounting surface . My thinking is that IF there is any inconsistency from one side of the carb to the other, it all mixes in the plenum.
You may be over thinking this to try and solve this by making changes to the carb.
Have you tried adjusting the idle mixture screws slightly richer on one side?
For your lean transition, have you tried closing the secondaries to 1/8 to 1/4" past contact of the adjusting screw.