[nhramnl]

I'm tending to support the "float too high" or "too much fuel pressure" suggestion. If the carb was fine but isn't now, your problem doesn't sound like a weather change issue. It sounds like a float that's too high (or the functional equivalent or that, too much fuel pressure) causing fuel to splash out of the bowl vent, into the primary venturis. If you leaned and richened it and the stumble wasn't affected, that would confirm that you have "uncontrolled" fuel entering the intake manifold. The car goes momentarily rich, stumbles and then gets on its way.