Yes, one of the main problems is in the carburetor. That's why I said "If you have the right carburetor." Last summer, I posted the following paragraph about the Rochester B/BC carburetors. I was (and still am) looking for possible ways to reduce the vacuum loss at the power piston...something "everyman" could do easily. Rochester wanted the piston to stay off the valve at 9 inches of vacuum or greater. If it would do that, there's a comfortable margin. The ones I looked at didn't and couldn't...at 11 to 13 inches the spring would push that power valve wide open. If you need more vacuum than 13 inches to keep the power valve out of play, I don't think there's any way that carburetor is going to give you good gas mileage unless you're driving on downhill roads with the wind at your back. And if your carburetor is running that rich, it doesn't matter what rpm you're turning. Efficiency is gone.
Here's what I posted...
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).
