Carter AFB Air bleed Mod

[Knebel]

Hi,

after debating if I should simply buy a new better Carb, I came to the conclusion that this is worth a try before shelling out $600 for a fully tuneable carb and I thought I share this here.

First, I put a Wire in my Idle Air bleeds and found out, that this gave my Idle Screwes a lot more responsiveness but in a very small range of 1/8th of a turn. It also got my off idle stumble taken care of. I have an AFR Gauge which showed me up to 17:1 with a very minimal throttle opening just above idle, holding it steady or not, this condition would NOT go away. A little more throttle and it would go down into the HIGH 15.9s... still not okay. With the wire in the Air Bleed I got my AFR down to about 13:1 to 14:1 and the car drives a lot better off idle now but now its more on the rich side.

My standard air Bleed size was about 0.058" and the wire I used was 0.021. I got me a 10-32 Tap, took the primary Boosters out and modified them for changeable Idle air bleeds.




The problem was, that there was not a lot of material to have them nicely sourrounded by the Venturi... and having the Air bleed sit on top wouldnt work because the Top of the Carb would cover it up. I went ahead and used the dremel to cut away the rest of the material and it turned out pretty nice, after that i realized i could have just used some brass set screwes with a hex and drill them my own. I had ordered some 0.070" Air bleeds which turned out to be too big but with the wire still inserted gave me a very good sensitivity on the Idle Mixture screwes and it got my off idle to almost 14.7:1, right where i wanted it. So now I have to figure out how to calculate the right air bleed size the right way without using the wire. I figured I need some 0.054" Air bleeds or so...

The mod is actually very easy if you are careful and pay attention to detail. I can just recommend this, it opens up a whole new world of tuning and I hope it also will reflect positivly on my MPG.

 

[70bigblockdodge]

Very nice keep us posted. If you took the area of hole/air bleed, radius squared times pi 3.14 minus the area of the wire, again radius squared times pi that should give you a number that you can work backwards to a diameter. I don't think there is a formula to get you there experience and trail and error.

 

[Knebel]

Update: the .070 bleeds with the wire are still too big. After driving a little i discivered, that I am still in the 15:1 afr range off idle and light off idle acceleration... will order some smaller bleeds and try those.

 

[Knebel]

I have just put in some 0.053 bleeds and it looks like they are working pretty good. I have some decent 14:1 ratios now and could probably go up to 0.054 or 55..... anyways, im gonna drive it a bit and see how it runs. I just dont like that it goes rich when letting off the throttle but i guess thats normal.

 

[67Monaco]

Well it goes rich because you have high vacuum and no air getting by the throttle blades.

 

[patinated_imperial]

This is gold (I think)! I can't believe I did not find this thread before. Found it on google.

I literally have the exact same scenario as you are describing on a 1965 AFB (model 3856S - 750CFM)- same AFR numbers, same "throttle situations". Went through different meeting rods,phenolic spacer etc. Car drives great except for that just off idle situation.

Are you still using this carb and the 0.053" bleeds?

What is your source of jets?

I am seriously considering copying your mod. I have access to dell'orto jets, which is in the size-range you have settled on.

 

[Knebel]

No i have long ago switched to an efi. The carb ran awesome after the mod. I believe I used either air bleeds from holley or possibly Power valve orfice jets. Dont really remember. Its a relatively easy mid.

 

[patinated_imperial]

Thanks for the feed back. After a little more googling using the right Carter nomenclature, I found a lot of good info on the speed-talk forum as well.
I went ahead with the mod yesterday. Ended up making my own air bleed jets. Since my secondary air bleed was already 0.053" (as you ended up with), I made mine 0.047" (Ø1.2mm). Testdrive hopefully this evening. Pictures will follow, if it is not a fail.

 

[Knebel]

Awesome! I remember I stuck some various sizes of wire in the bleeds and testet it. I then calculated the area of the bleed and the wire to get a rough size of the airbleeds I needed.

 

[patinated_imperial]

Home from first short test drive after changing the air bleeds from 0.053" to 0.047".
Before, I had set the idle mixture screws to AFR 14.2. With the new air bleeds it idled at AFR 11.8. I adjusted the mixtures screws to AFR 13.2, as it seemed to result in the highest idle rpm. The lean spot is still there, but it is fatter (~15.5) and harder to find . It might only be noticeable because I have the AFR gauge. Maybe I will try Ø0.0433 (Ø1.1mm).
Still find it odd, that AFR seems to be "pulsating" right at that lean spot.

This is just "kissing" the surface with a step bit (unsure of this is correct english word), to make sure it is perpendicular to the thread and flat.

 

[Knebel]

Do you already have everything else dialed in? Springs, crusise AFR and such? The airbleed mid was the last thing I did because after I had everything dialed in perfect, just off idle id get a lean stumble that I could not tune out.

Those bleeds will affect the crossover from the transition slots to the main jets!

 

[patinated_imperial]

I think I do... Actually I should go and update my original thread about this problem (where you have also replied). Will do in the coming days.

Also I do believe my problem is right at the crossover from low speed circuit to main (primary). This mod is probably the biggest improvement of all I did. Need to do some more test driving though.