So, the old float languished beneath the heat lamp I use for drying out clock parts for about an hour, and then sat overnight under cooler conditions. It looked like most of the 'outgassing' occurred through the holes that the needle goes through, but I'm not sure. I'm now going to dip the float into Red-Kote gas tank sealer and let it dry for a day or two, then try it. I shall report on my progress.
I also managed to lose my idle control screw, which turns out to have a M5-0.8 mm thread. So a bolt fits in there nicely, but I forget: is the tip of the screw ground to a perhaps about a 60 degree point, like a center-punch would be?
Thanks for your continuing help and patience.