You are already too lean, you need to richen the mix more. Cold air needs more fuel. I know it sounds counterproductive. Cold air is more dense so more fuel is needed. You may have to loosen the adjustment to get more range out of the needle valve. I assume it has the stop so you can't over adjust it? Loosen the set screw and then you can turn the needle valve itself.
Oh, wait, I'm thinking of the idle mixture screw.
In any case more fuel is needed. Richen it until it blubbers and then lean till it cleans up, then a tad back rich. If you have a tach, set it 500 to 600 rpm to the rich side of top rpm.