[jhv48]

Your problem is with the carb, not the plugs.

It sounds like a fuel percolation problem. Somehow, you've got to insulate that carb from the engine temp during the winter months when our gas turns to crap.
You can always get a drop base air cleaner to fit under the hood so you can use a phenolic spacer to help insulate the carb.

Also, try opening the hood as soon as you get home and leave it open. Helps dissipate the heat quicker.

And, finally, most newer carbs have blowout proof power valves, so I doubt that's your problem.