Day before yesterday I made a small T so I could test (with a known good vacuum gauge) the Rochester carburetors I have with the o-rings on and off of the power pistons. Everything can be condensed into 4 points:

1. The o-ring with grease in the center scheme works and does not alter the designed vacuum points.* In fact, I believe it is the most logical and affordable way to restore the designed vacuum points and make these work as designed. Because of this first and main point I'm not considering any other approaches to the "Power Piston Problem." Good or bad, the o-ring answer is as far as I'm going to take it. However I've made a change or two and I will post that along with some other suggestions soon.

2. The wear of time and use causes these carburetors to run more richly. Why? Couple of reasons. The constant loss of vacuum through the worn power piston bore will be between 2 inches at the best and probably more like 3 or 4 inches. For all I know it could be more in some cases...I can only judge from the ones I have here. This lost vacuum prevents the piston from working as designed. It was designed to drop down and cause the power valve to activate in Rochester's own words "at approx. 9" hg." When it was new, I'm certain it did. Today with the wear I've observed and testing with vacuum, that piston could drop down at anywhere between 11" and 13" and that puts the power valve into play at least 20% sooner than it was designed. The flip side...with the wasted vacuum, your power piston will require more vacuum to lift it back up and off the power valve. And the more the carburetor wears, the worse this will get. The o-ring approach gets us very close to the 9" Rochester envisioned...actually a bit less (which is on the good side if you're hoping for a leaner mixture).

3. Without the o-rings the pistons tend to run a little wonky. The spring and vacuum are opposing forces and the combination of these plus the worn bore seem to set up odd patterns of motion...not fluid motion but jerky motion. With the o-rings, the slack is removed, the motion is dampened and fluidity of motion is restored.

4. As I said, the o-rings dampen the sideways movement of the piston in the bore. They also reduce lost vacuum to a trickle**.

However I will tell you that today I received the other o-rings I bought and I now know there is only one acceptable size. Any larger and the piston sticks. Any smaller and you may as well have nothing there.

* assumes the piston spring was the correct one for the carburetor. Important note: of the 5 I've got one is 25% weaker and some others are weaker by varying degrees. I can't unscramble that egg, but I do know there were different strengths of springs used and obviously this is important as the strength of the spring determines the piston's timing.

** assumes there is no vacuum leak at the air horn where it joins the float bowl. Please see the work on straightening the air horn I posted a few weeks ago.

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