New to Holleys. I've just put a six pack on the engine, and A/F at WOT is in the mid-low 12's, cruise at low 13's. Seems ripe for a jet change. I have 66's in there now, and a set of 62's, is that too far to go to try and move that WOT up to about 13.0:1?

iirc someone priorly posted that 12.5 is a good number for WOT? but wait for guys who know more on that

I would look to make the motor leaner first by leaning the idle mixture screws down on all three carbs., then go from there
My old pump gas six pack liked #63 jets in the center carb and 1/4 to 1/3 turn out from gently bottomed out on the idle mixture screws on the outboard carbs and 3/4 to 1 full turn out on the center carb.
What carb. List # are you working on? If they are replacement carbs. (-1 or -2) your probably dealing with a really lean set up from Holley for smog. Have you enlarged the outboard carb. main fuel feeds in the plates? My carbs ran the fastest ET and MPH with 13.3 AFR at WOT above 5500 RPM
It FELT better at 11.8 to 12.5 AFR

With all the comments in your initial post,unless I missed it you never stated what cam,as far as 62's I think you will be lean with a 540 cu in motor at WOT.As far as what Cab posted,the replacement carbs are exceptionally lean and I think you stated you never did anything with the outboards.

I expected to be very lean as well, but the A/F meter is showing otherwise. I put 66's in as a recommendation from someone who know's six packs, although I really should have left it alone with the 62's and just started with that as a base.

The cam is a solid roller, 248/255 .604/.582, 112 LSA, installed at 110 Centerline.

For this engine I want to be 12.8-13.2 at WOT, so whatever it takes to get there.

I will check again, but last time I had the idle doing well. Smooth idle, outboard carbs adjusted correctly, i.e. cover up the transfer slot and no change in RPM.

Check your spark plugs to verify the meter isn't being fooled.
Yes, with a 4 bbl, 2 jet sizes is typical change.
On the primaries, or in your case the center carb, jet change should effect high speed cruise AFRs. High speed = Interstate speeds, around 60 mph and up depending on gearing and incline.

When you say outboard carbs adjusted correctly,what did you actually do ?

Per Dayclona's advice, I plugged the outer air bleed on each side with my finger. If the rpm rose or fell, which it did, I adjusted the idle mixture screw until there was no change when I plugged the hole.

Why? Wouldn't you want to jet for max power at WOT?

Go back and adjust the outboard idle mixture screws in 1/4 to 1/2 turn out from gently bottom with no other changes and then go drive it and see what that does the cruise AFR reads
AndyF turned me onto tat trick accidently, THANKS Andy
the idle AFR affect all the way from idle RPM to WOT

This is basically one of Tim Banning's engines. He runs his stuff there, so that's what I'm working with.

Where do you run yours for max power?

Your reasoning is rational. It just came across as if you are tuning to AFR instead of tuning to torque and power. Differences in load, fuel distribution, and fuel itself are some of the things that can alter AFR that your engine appears to run best at.

One reason your engine may need to be richer is to provide enough fuel to take care of the leanest cylinder. If you know which cylinders are richer or leaner, then you can try to improve the distribution.

Another is that AFR will also vary with fuel composition and density. Race gas is held very close to whatever the manufacturer publishes. Pump fuel will vary from year to year, region to region and winter to summer.

(The AFR gage is an interpretation of a WBO2 which itself is an interpreted measurement of oxygen. There are conditions that will result in misreadings. Another reason to check the spark plugs.)

If you are running the current Holley replacement carbs,the # 35 metering plate with your cam and the 540 cu in,I think you will still be lean at WOT.Buy a set of Ouick Fuel cast plates,$25 ea,take your time and eventually you'll get there.