Off idle lean condition, suggestions?

[Cadman-iac]

  So I've got this carburetor pretty much dialed in, the idle is good, AFR at idle is right around 13.0 to 13.4, and the  cruising AFR is great, even when towing a heavy trailer.  I have #71jets with Cliff's #44 rods and I'm using the red spring for the PP, so everything looks good across the board, EXCEPT  for the unloaded off-idle AFR mixture, it consistently goes to 16.0 to 18.0 + depending upon the engine temperature. Under load, meaning in gear this is just a momentary thing as the engine speed is increasing as you are taking off.
 The speed in question is just above the idle of 600/650, beginning at 700 and up to 1000rpm the AFR goes extremely lean in a free idle if that's the right term.

 I'm using a 17057230 from a 77 Cadillac 425 on a 350 Chevrolet in an 88 3/4 ton Suburban and I've increased the idle by-pass holes to .068, increased the idle tubes to .040, changed the channel restrictions to .050, and the mixture screw holes to .078, which has significantly improved the off-idle lean condition, the range is much narrower than it was, but it's not gone yet.  And I have raised the APT as well, I had it set at 2 turns up, and have gone as far as 5 1/2 turns, but it runs best with it at 4 turns in all areas except for this narrow range and conditions.
 Just how far can your AFR go towards lean before it begins to cause damage to the engine?
 I must state that this is a really narrow problem band on an unloaded engine which it probably won't be in for very long. However, that said, it does occur
worst while the choke is still engaged and the high idle cam is keeping the rpms in this range. Once you kick it down and the high idle cam is no longer in effect, the AFR drops back into an acceptable range, usually between 12.5 and 13.5 depending again on engine temperature. You can still raise the rpms and see the AFR go lean once the engine is hot, but I'm more concerned with this as the engine is warming up with the choke keeping the rpms up and I might be elsewhere as it's warming. It's what's going on when I'm not looking at it that I'm worried about.

 What effect would changing the lower idle air bleed have on this small window of operation? Is this even the right area to be focused on, or should I look elsewhere to correct this issue?
 Any and all advice is welcome and appreciated.

 Rick

[77cruiser]

Does it buck or surge?

[Cadman-iac]

Does it buck or surge?

 No, it doesn't do any of that, but it's only a free idle, no load situation,. At the same rpm with a load it has a slightly leaner AFR but not as bad as the unloaded AFR is, and loaded it's at an acceptable range, plus,  this is a range that I don't expect to be in but momentarily when it's in gear.
 I've got an AFR gage installed to keep an eye on it so I don't damage it unknowingly, and it fluctuates with load and rpm as you would expect.  It goes into the rich range as much as 10 5 in certain situations and lean as much as 16.5 at times as well but these I understand taking the load, speed and throttle position into consideration. It's just this one parameter that I can't seem to adjust out of it.
  I would like to be able to start it and let it warm up on its own and not have to worry if it's running too lean. Right now I have to keep an eye on it and kick it down as quickly as the choke will allow, which is actually pretty quick, but if I were to go back in the house for a few minutes it would keep running really lean until I tap the throttle and drop the rpms.  You can hear the difference in the exhaust note when it's lean, it's not as crisp if you know what I mean, and you can almost hear it wanting to misfire but it's slight, like it wouldn't take much to make it misfire.
 
 Any ideas on what to adjust or change to correct this?

 

[77cruiser]

If it runs fine I would worry & it's not going to hurt anything being lean at part throttle.

[quadrajam]

^^^^ what 77 said.

QJ