Well, sounds to me like it is broke in cold temps... here's your big chance to dig in gracefully.

If your carbs have small screws with locknuts set at an angle outboard of each barrel -- these often have white plastic caps on them -- undo the nuts and make sure they are turned all the way in. Don't force them. Lock them back down. If you don't have these, great.

Idle mix screws are the rearmost ones outboard of each barrel. Get everything warmed up and the plugs unfouled. Stop the motor. Turn the mix screws all the way in until they stop gently on their seats (forcing them tight is the surest way to ruin a Weber). Count 1/2 turns, just so you have an ideas of where they were to begin with.

Turn them back out 2-1/2 turns. Start motor and let it idle. Turn one screw in 1/4 turn and listen for a change in rpm/tone. Turn in another 1/4 turn if necessary until the idle degrades. Turn back out in 1/4-turn increments until the idle speeds up. Repeat for the other screws until you have the fastest/smoothest idle.

All the screws should end up within 1/4 turn of each other. If best idle is three or more turns out, the idle jets are too lean. If less than two turns, they are too rich. (Note: This is for DCOE carbs; DCO uses a coarser thread on the screws and they shouldn't be that far out.)

The idle screws affect the mixture only when the throttle is closed, but that's regardless of engine speed. If you're going down a hill at 3000 rpm with your foot off the gas, it's still the idle screws that control the mixture, regardless of what jets are in there.