Lean spot from 1800 to 2,500 light throttle

[lluciano77]

I have a wideband 02 sensor, a vacuum gauge in the dash, and a tach. I know exactly where my engine goes lean....1800 to 2500 RPM. This happens in drive (3rd) with light to moderate accel. My fuel curve is right on the money everywhere else. At the above cruise RPM I am usually around 16" of vacuum. Distributor curve is good as well at 20 initial, 34 total, and basically all in at 3,000 RPM. With a 3 step dist. curve. I have vacuum advance set at 5" via manifold vacuum and all in by ilde vacuum (12"). My torque converter is a 2,500 stall.

My carb is a Demon Annular 830. I am using the lower IFR locations. Metering blocks are at Holley 4 hole spec. Primaries are at .020" showing and square below the blades. I am running a 6.5 PV. Jetting and PVCR area is good all around. At least under full throttle launches.

On the wideband 02 the fuel curve doesn't richen up when the above problem happens. It stays roughly at 14.5:1, untill I pass this range or dowshift and then goes to a more healthy 12.5 under heavier load.

My IFRs are possibly smaller than they should be. I know last time I messed with the carb I brought them down, and enlarged the IABs to get my idle right. I don't have all the sizes...will have to measure this weekend. But I am guessing the IFRs could be at .028"

So with the above known. I was thinking of running a 10.5" PV. But I don't want to waste gas in other conditions. Where is the most likely place to cover this gap? Larger IFRs? The 10.5 PV? Or could this be an MAB problem not bringing my main circuit in in time?

[zums]

if your sure your at 16" vac in that range a 10.5 wont do you much good, with that stall and vac reading you should be able to smooth it out with an increase in ifr and additional idle bleed tuning

[lluciano77]

The pv would be a stretch as vacuum is high at cruise.

[levisnteeshirt]

a 10.5 would help a lean spike ,,, they makes a 12.5/5.5 double if you want to split hairs

[lorax]

No wonder DV's new carb book doesn't delve into emulsion. There is still a total misunderstanding of the PV to conquer first!

[Roadknee]

Sometimes an engine will want to run richer under those conditions, or use more advance. As an experiment, bump up your initial timing 10 degrees and see if it helps. Don't go WOT though. If it works, adjust vacuum advance accordingly.

[GARY C]

No wonder DV's new carb book doesn't delve into emulsion. There is still a total misunderstanding of the PV to conquer first!

See you have the option of writing a book, answering the OP's question or making a smart ass comment...some don't have that option.

[lluciano77]

The starter wants to kick back any more past 20 initial. If I add more vacuum advance it will ping in the situation above.

[lorax]

No wonder DV's new carb book doesn't delve into emulsion. There is still a total misunderstanding of the PV to conquer first!

See you have the option of writing a book, answering the OP's question or making a smart ass comment...some don't have that option.

The only info I can offer is that he not going to fix a LIGHT throttle lean, pulling 16" with a PV.
Do I have a fix for a steady 1800 RPM lean transition to 2500? NO. I will let Mark and Tuner chime in. But my money says neither are going to recommend FIXING it with a PV
Now wouldn't it me interesting if either even remotely mentions ABs, emulsion, or transition restricters? MMMmmm

[lluciano77]

I think the issue is more with carb timing or enrichment. Dist. Curve doesn't have more room for advance at my 11.25:1

[zums]

who said anything about fixing it with a pv

[lluciano77]

If pv is not an issue... my question would be does the idle circuit need to fill this gap or the main circuit?

[zums]

guess you guys cant read

[lorax]

guess you guys cant read

You sure its us?

[n5ifi]

I have a wideband 02 sensor, a vacuum gauge in the dash, and a tach. I know exactly where my engine goes lean....1800 to 2500 RPM. This happens in drive (3rd) with light to moderate accel. My fuel curve is right on the money everywhere else. At the above cruise RPM I am usually around 16" of vacuum. Distributor curve is good as well at 20 initial, 34 total, and basically all in at 3,000 RPM. With a 3 step dist. curve. I have vacuum advance set at 5" via manifold vacuum and all in by ilde vacuum (12"). My torque converter is a 2,500 stall.

My carb is a Demon Annular 830. I am using the lower IFR locations. Metering blocks are at Holley 4 hole spec. Primaries are at .020" showing and square below the blades. I am running a 6.5 PV. Jetting and PVCR area is good all around. At least under full throttle launches.

On the wideband 02 the fuel curve doesn't richen up when the above problem happens. It stays roughly at 14.5:1, untill I pass this range or dowshift and then goes to a more healthy 12.5 under heavier load.

My IFRs are possibly smaller than they should be. I know last time I messed with the carb I brought them down, and enlarged the IABs to get my idle right. I don't have all the sizes...will have to measure this weekend. But I am guessing the IFRs could be at .028"

So with the above known. I was thinking of running a 10.5" PV. But I don't want to waste gas in other conditions. Where is the most likely place to cover this gap? Larger IFRs? The 10.5 PV? Or could this be an MAB problem not bringing my main circuit in in time?

Is the T slot Restricted? What size? This may be the problem.
What is the Idle Feed Restriction?
Is there a kill bleed in the block? Size?
Which Emulsion Bleeds are used and what size are the restrictions from top to bottom.(example .028,blocked,.028,blocked)
The power valve may or may not be right but it isn't the issue here.
That should set up Jmark or Tuner.