Hi Gord. I haven't given up on the o-ring idea, but I want to come to a solution that leaves as much of the carburetor "as is" as possible so that "everyman" could get a kit and use it and hopefully improve things for himself or herself.
At the risk of being boring, please let me wade out a bit more. The choice of o-rings was by no means accidental. I measured everything about 27 times before I ordered them. If you look at this digital image of the piston, you'll see the o-rings appear to be the same size as the piston or possibly a bit less. In fact, they aren't but only .006" of the outer edge of the o-rings touches the bore of this carb I'm using as the test case. A typical sheet of copy paper is .004" for reference.
http://i1154.photobucket.com/albums/p535/JonGoodman/Rochester%20O%20ring%202.jpg So as I said, when the piston is in the bore, only .006" of the circumference of the o-ring touches. Admittedly, it does slow the piston's movement a bit, however this is dry and the resistance is not as much as anyone might expect. I haven't tried any lubricant yet and that should make a large difference. It actually seems that the piston operates at the same vacuum range as designed.
Based on Jon's comments, I'll try to look for a hard (but gas resistant and slick) material that could be used in those grooves instead of an o-ring. It will have to be a split ring like a piston ring, but maybe something exists.
