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 Once you kick it down and the high idle cam is no longer in effect, the AFR drops back into an acceptable range, usually between 12.5 and 13.5 depending again on engine temperature.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

[novadude]

There is absolutely no concern with being lean at part throttle / light load if it is not causing drivability issues.  Zero engine damage will occur. 

In fact, every text book about engines written in the "pre-emissions" era suggested a target A/F of 16-17:1 for best economy if the engine will tolerate it without misfire/ surging. 

Personally, I've dialed on my own Q-jet to go down the highway at 15.5-16:1 (level load, steady cruise).  The only time you need to worry about "too lean" is under heavy load.  Mine will trend even leaner (high-16s) as you lean into the throttle, until you get the point where the power piston spring overcomes vacuum and lifts the primary rods.  In my case, it took some experimenting with PP springs to get there.

Lean keeps the oil and spark plugs cleaner, and also has the benefit of keeping more gas money in your wallet.  LOL

[Cadman-iac]

  Thank you gentlemen,

 I appreciate your support. Novadude, thanks for the explanation about the acceptable range for the AFR during certain conditions. I had noticed that it would lean out as the throttle tipped in and then would richen up as the power piston raised.
 What is really worrying is when the AFR drops beyond what the gage can register at 18.0 and I can tell it's starting to misfire from being too lean at that point.
 I've been trying find the specific parameters where this occurs as it's not often, but I have seen it a few times.
 It's mainly when I'm accelerating slowly, but I've got to do it a few times to make sure I can duplicate it and explain exactly what the conditions are. 
 When I first posted this question I had not noticed this particular situation, I thought I only had an issue with the initial startup with the choke on.
 But at least I now know that a 17.0 with a light throttle is nothing to worry about. Thank you for that.

 Rick

[Cliff Ruggles]

In any and all cases tune for results vs what a gauge is telling you.  At very light engine load nothing at all wrong with being lean provided there is no power loss, engine miss-fire, going "flat" on power production, etc. 

I'd be interested to know what the distributor is doing thru all this and how it is set-up.  It's a big player in tuning, especially at light engine load as it uses a vacuum advance to add timing to effectively burn a lean mixture....

[Cadman-iac]

Cliff,

 I've got the distributor set up for a maximum of 41 degrees advance, 10° initial, 16° mechanical, and 15° vacuum.
 The mechanical is all in at 3000 rpm, and I'm using manifold vacuum to the advance unit. It's an HEI unit.
 I just ran a 200-mile round trip to Tucson and back yesterday, and it seemed to be fine except for a slight fuel starvation issue due to the stuff I have in the supply line. I've got a shut-off solenoid and an electric pump about midway between the tank and the mechanical pump. When I noticed the problem, I turned on the electric pump, and that solved it.
 I think I'm going to relocate the solenoid to above the mechanical pump, not my preferred location due to it being seen easily, but better than letting it cause a problem. (Down here on the border; it's necessary if I want to keep it in my possession).
  I did have a  heat issue coming back because I was running the air conditioning, but it never overheated, just increased by about 15-20 degrees. I'm running a 180 stat, and the gages registered the normal temperature for when it had a 195 stat, so I'm attributing it to the extra load and the steady rise in elevation between Tucson and here.
 The engine has just over 1350 miles on it now, still using a slight amount of oil, I believe, due to the Moly rings. I pulled a couple of plugs just to check and they looked great, a slight brown coloring in spots, which I believe is normal.
 I did notice that while cruising at 75, at times the AFR would run upwards to 16.0, but if I gave it a little throttle it dropped back down into the richer zones. I'm getting used to seeing the gage fluctuate a lot and I'm paying attention to the circumstances at any given time.
 Pulling, accelerating, the AFR is always in the 10.5 to 14.0 range, depending on how many toes I've got shoved into the carburetor.
 
 On the distributor specs, is what I've provided enough, or do you need more specific details? If so, please let me know and I'll find out and post it here for you.
 Thanks for your help and expertise  I really appreciate it.

 Rick

[77cruiser]

You need about 10 more degrees. Put the initial up to about 15.

[quadrajam]

I agree, up the timing some. If that's too much @ idle, switch to ported vacuum.
Easy to try.

QJ

[Cadman-iac]

  Thanks, I'll give that a try and see what happens.

[Cliff Ruggles]

You may need to run a little more total timing but it's not always best to increase initial timing to get it.  Too much initial often results in difficult hot restarts.  For most well thought out engine builds I like to use around 10-12 initial, 10-11 mechanical (20-22 at the crank), and 10-15 degrees from the VA.  In most cases I use ported vacuum to the advance as my engines are higher compression wider LSA camshafts which promote good cylinder pressure at low RPM's so no need to run a butt-load of initial timing to make the engine happy.

When I custom tune engines brought here I use similar numbers unless they are lower compression with big cams or tight LSA.  I always address idle fuel delivery as well and when I get that up to par it's still rare to need a lot of timing at idle speed for most engines. 

Anyhow, it is best to make very small changes with total timing.  Right now you have 26 degrees total timing at full load.  Most engines I've tuned like closer to 30-34 degrees with some liking a bit more than that.  Some engines like a tad less so be conservative when advancing the timing as detonation may occur and it will destroy the best prepared engine......