For a while it has been on my mind that the power piston in the Rochester B and BC models is one of the main problems. Well actually for these carburetors I believe it is more of a time bomb than anything. These pistons always get stuck in the down position, and this invariably makes the carb run very rich. Straightening the air horn and bowl helps. Adding a brass tube between the two seems to help strengthen the vacuum passage, but that probably isn't enough. Why? The problem is a chain of events. I believe the worst of things is ethanol gas and infrequent use. The gas when it dries leaves a sticky residue. As it ages, this gas devolves into a fluid which in concentrated form leaves more of a sticky residue...almost rosin-like or something you might use to make flypaper. The power piston is a close fit in a bore in the air horn and pushed ever downward by a spring. When the carb dries out (as it will in about 1 week of non-use), the residue acts as a bonding agent between the piston and the bore. In one of the 5 Rochester carburetors I recently studied, the piston was stuck so tightly it took me quite a bit of time and work to remove it without damage...it was stuck nearly as if it had been glued. So what is the answer? I think it could be as simple as keeping the piston from sitting against the bore by using a small o-ring like this one:

http://i1154.photobucket.com/albums/p535/JonGoodman/Rochester%20O%20ring.jpg

Conveniently the piston has two slots machined into it on top and bottom. But the o-rings must be small enough they don't interfere and yet large enough they are slightly larger than the piston itself like this:

http://i1154.photobucket.com/albums/p535/JonGoodman/Rochester%20power%20piston%201.jpg

So far, vacuum tests prove this approach works just fine. I'm thinking since the o-rings will more or less seal the center zone of the piston, some light grease (maybe silicone grease) will improve the scheme further.

Thoughts?

PS...Jon (King of Carbs) I know you're scratching your head saying "seems an awful lot of time to spend on a Rochester B." Probably so.

~ Jon
1952 1/2 ton with 1959 235 | T5 with 3.07 rear end