Part throttle cruise air fuel ratio very lean on a thermoquad

[femtnmax]

I'm adjusting the air fuel ratio on my thermoquad, carb part #6322 off a 1973 440.   I can adjust the air fuel ratio at idle, ended up with 13.5-14.5 .
The problem is at part throttle cruise the meter shows the ratio is over 15:1, yet the engine runs well with a little lean surge only when the engine rpm's drop below 1300 rpm in gear.  During firm acceleration the ratio is 12-12.5, but as soon as the engine speed is constant the ratio goes "off the chart" meaning leaner than 15:1 . There is NO lean surging at cruise.  During cruise just give it a little gas pedal and acceleration is right there right now.

Just above 2800 rpm the stock jetting was good at 13.5-14, though the air fuel ratio was jumping around a fair amount from these numbers and higher.   I tried richening the primary jetting one step.  This change did not do anything but make the main metering ratio drop to 13:1, which is too rich for cruise in my book.  Note the primary jets don't come on until about 2600 -2800 rpm, so below that the engine is running off the idle/transfer slots. 
I wonder if its the RPM air gap intake manifold?  They say these need a richer mixture to overcome the lack of heat.  I retorqued the intake manifold and header bolts.  Did not see any exhaust gas leakage at the collector flange.  The O2 sensor is located just downstream from the collector flange.  There is no engine knock on acceleration.  Piston to head quench is 0.045 inch.  Timing 14 degrees advanced at idle with 38 total by 2800 rpm.

The car drives well...at this point I guess I'll call it good enough, though I'm a little concerned that I can't dial the mixture in the way I'd like to.   Even the Edy AVS carb on my old truck was totally tunable with the meter.
Any thermoquad comments or experience??  Thanks.

I have an old Carter AVS off a late 60's  440, maybe I'll put a kit in it and give it a try.

[Bullitt-]

I have no experience fine tuning a T/Q with a meter but FWIW a local club member uses an O2 sensor on his 440 with a long duration cam. The cam lowered the vacuum quite a bit so he was having problems with the metering rods not staying down with the spring the carb. came with. He discovered that some T/Qs, have lighter springs and found one that allowed the rods to stay sucked down. I suppose you do know that turning the screw in the metering rod assembly will change the mixture.   

[femtnmax]

I suppose you do know that turning the screw in the metering rod assembly will change the mixture.

Yes, I tried changing the metering rod tree adjustment, but did not see an improvement in my off-idle/low speed cruise lean condition.  Good info on the various tree springs, I did not know they had different spring rates, though it makes perfect sense.
I was taking all my vacuum readings off the hose fitting for the vacuum dashpot, but come to find out there is a restrictor in the passageway, so the vacuum readings were 4 lb low.  Feel kinda stupid not spotting this earlier.  Live and learn.
I thought about the lean condition some more...one other piece of info is the idle screws have to be turned out 4 turns, which is more than I'm used to seeing on these old carbs.   So if the idle mix screws had to be set rich, and the low speed is lean, maybe some of the air bleeds in the air horn assy have been drilled out, resulting in a lean condition.  They might have drilled them out to make up for a rich condition.   
I'm going to swap in a different air horn assy and see what happens.  I'm also rebuilding the old Carter AVS from a "69 440, info says its 750 cfm, and its pretty much a square bore carb.  We will see which one works the best with my engine/ gearing combo.  Guess I like to tinker.

[ShelbyDogg]

I don't think anything over 15.1 is too high for part throttle cruise. Frome my Holley fuel injection manual:

Stock and Moderate Performance Engines
Idle and Cruise: 14:1-15.5:1
Moderate Load: 13:1-14:1
Wide-Open Throttle: 12.2:1-13:1
Large Cams and Race Engines
Idle and Cruise: 13:1-15:1
Moderate Load: 12.5:1-13.5:1
Wide-Open Throttle: Typically 12.5:1-12.7:1 at peak torque and 13-13.2:1 at peak HP
Supercharged/Turbo-charged/Nitrous Applications
Boost Conditions: 10.8:1-11.8:1 for safe operation

Sounds like you are right in the ballpark for getting good gas mileage.

[the_engineers]

What meter are you using?

[femtnmax]

I don't think anything over 15.1 is too high for part throttle cruise. Frome my Holley fuel injection manual:

Thanks for the Holley info.  Using an Edelbrock air fuel meter, it only goes to 15:1 ratio.  During the lean condition the 15:1 light never flickers, and the engine runs just slightly rough, so my guess is the mixture is over 15 by some amount.  I would feel better if there was no roughness, and if the 15:1 light flickered occasionally.  I'll work toward the Holley specs.  thanks.

[Skunkworks Challenger]

Hi Phil, I used the advise you gave me when rebuilding my thermoquad and it works great.  I've not tested for air/fuel ratios but with the highway mileage I'm getting it must be very close to correct.  The car has lots of power and no flat spots.  I couldn't be more pleased.  Being the professional you are I'm not surprised  you are going for optimum performance, both in economy and power.  Thanks for your recommendations!  PK