AFR dropping just off idle

[Cadman-iac]

  What would you do to correct a lean condition at just off-idle, or just as the throttle starts to open?
 Would a larger down channel restriction fix this, or a bigger upper idle air bleed?

 Running the mixture screws out until the AFR is rich still doesn't correct the lean condition at throttle opening, nor did raising the APT screw.

 So which would you change, upper idle air bleed, down channel restriction, or idle tube size?

 This occurs in gear when opening the throttle to check the AFR during the transition to the main circuit. Idle speed set at 550 rpm in drive, and the lean condition starts at 650 to 900 rpm, too low for the main circuit to begin working.

 Thanks for any advice, Rick

[Kenth]

I would set idle speed at 650 rpms in drive, then adjust the idle needle screws for best vacuum/rpm.
Reset the idle speed if nessecary and check the idle needle screws last, both the same amount of turns out when finished.

If that does not help, start with open the idle tubes .002", downchannels .003" and check for at least .086" idle needle holes.

HTH

[Cadman-iac]

  I increased the idle in drive to 650 and adjusted the mixture as suggested but it made no difference.
 So I increased the idle tubes by  .003" and the down channel restriction by .004" and left the idle ports at .065", then dropped the idle back to 550 in drive. This improved the off-idle lean condition quite a bit but it's still a little lean yet between 600 and 850 rpm in drive.
 I didn't want to change the idle ports as it's not reversible if ineffective, the idle tubes and channel restrictions can be reversed if necessary.
 So with that done, do you think if I increased the two areas I've already changed would help eliminate the problem or should I try increasing the idle ports?

 Would bigger bypass air help here?  That is currently at .039". My thought is it wouldn't as it doesn’t increase the fuel availability any, only the air so may make the problem worse, but I thought I should ask anyway. What do you think?

[Kenth]

I have found that .086" is the needed minimum size on the idle screw ports for most performance build to not have the screws falling out of the threads.

How many turns out are the idle mixture screws?
Does the rpm decrease when turned out further from best vacuum/rpm setting?

[77cruiser]

When I put the IAB in the top they can be changed without pulling the carb apart. Makes it handy for these kinds of cases. But if you want to start modifying to that extent I understand.
It's easier if you have a couple of junk cores to practice on before hand.

[quadrajam]

You can go ahead with .086 on the idle mixture holes and if thats too much
you can screw the mixture screws back in some. same/same

QJ

[Cadman-iac]

 Didn't get a chance to work on it yesterday,  I'll try enlarging the idle ports today and see what happens.

[Cliff Ruggles]

If you are working with a later model carburetor with a big lower IAB you can shrink that bleed down slightly to put more fuel where you need it without increasing IFR's or DCR's.  I do this on some of the later model Buick, Caddy and Chevy units to correct that issue.....

[Cadman-iac]

  Cliff,
 I just posted a few questions in another thread on this 17057230 carb about inconsistent AFR's between idle and high idle as it warms up. If I shrink the lower idle air bleed size, you're saying that would level out the AFR through the lower rpm range? Or did I misunderstand what you meant?
 They are at .078 right now, which is the factory size, so if I wanted to shrink them, how would I go about it?

[novadude]

  Cliff,
 I just posted a few questions in another thread on this 17057230 carb about inconsistent AFR's between idle and high idle as it warms up. If I shrink the lower idle air bleed size, you're saying that would level out the AFR through the lower rpm range? Or did I misunderstand what you meant?
 They are at .078 right now, which is the factory size, so if I wanted to shrink them, how would I go about it?

I think that is what Cliff is saying.  From my q-jet experiments in carburetion, I've found that they seem to run better with lower IAB in the 0.062 -.067" range.  For me, it seems to help smooth the off idle.

In theory it should also deliver less fuel at heavier part throttle, since this air bleed tends to become a fuel feed when the throttle blade edge gets close to the bleed. 

I've used a punch and an old accelerator check ball to shrink the bleed, and I typically qualify the hole with a #52 (0.0635) drill after swaging it smaller with the punch/check ball.  Curious to hear if others have had similar experience? 

[77cruiser]

I tapped mine to 6-32 & did the setscrew deal.

[Cadman-iac]

  I just realized I gave the wrong size on my lower idle air bleed. It's actually at .048, not  .078. The. 078 is the mix screw hole size.
 So what would I do now, increase the bypass air holes, they're at  .039 which is the original size for a 425 Cadillac.  If so, how big should I go?

 Rick

[Kenth]

I have found that 1977-79 Cadillac 425 units has about .078" lower air bleeds and about .049" idle needle holes.

If the throttle blades are in a good location at idle speed and no nozzle drip occurs there is no need to enlarge the idle bypass air supplement.

FWIW

[Cadman-iac]

 No, there's no nozzle drip, I've checked it for that and to see when it begins to pull from the main system.  I don't remember exactly when it begins pulling from the main system, I want to say around 1200 to 1300 rpm, but I've slept since I checked and my memory resets every night after the reboot.
 As for throttle position, I haven't tried to see how much farther it will close, but if it's not there now it's almost there, so I don't think that's an issue.
  I did enlarge the mixture screw holes to .078 to try to eliminate the off-idle lean condition it had. That's not nearly as bad as it was now.
 The more I drive it and watch the AFR gage through the various rpms and loads, the more I think I understand what is going on. I just want to be sure I'm not misinterpreting what I'm seeing.
 I couldn't really tell how fast the engine was running because my old tachometer was bouncing and I couldn't get it to clear up, so Tuesday I got my new tachometer from Summit Racing (made in America)  and I can tell much better what's going on.