If your Weber has an automatic choke, that can possibly be adjusted to idle better when cold. I found that the one I had would freeze up terribly in the cold and/or wet, even with the radiator water hooked to the manifold, so that might be your problem as well.

I can't say what's wrong with the SUs you have, but very fast idle is due to unwanted air getting into the intake. A good rebuilt set can run extremely well when set up properly, and require no fussing around at all for a year at a time.

I've gotten excellent SU rebuilds at a sane price here:

www.sucarburetors.com