My holley 4160 style carb is almost pefect in all respects ( I run with a LM1 hooked up) except at low speed cruise.

The problem is at cruise 30 -50mph very little throttle is required and the F/A runs lean 15+,
I can improve this a little if I richen up the idle, but I don't want to do that as it idles nice now at 13.5-14.1 without excess fumes.
If I want to increase my speed slightly while cruising and just breath on the peddle a little, it goes even leaner, so I have to give it a little more (more than I want ) then it richens up to a perfect level. Then I'm going faster than I want to and have to slow down.
This takes place usually in the 1500-2300 rpm range, above that the ratio is usually OK. The F/A is within range in all other circumstances and the carb performs flawlessly otherwise.
I know there is 1 more adjustment in there somewhere that will cure this But I can't figure it out.
Any help would be appreciated as I don't like it running this lean while I am cruising--which is most of the time unfortunately due to our rediculous anti- racing laws.
Thanks Craig