[Dan Case]

I don't know much about the R-3255 series of carburetors but the functional changes in the R-3259 series for 289 engines was aimed at improving all around drivability and performance. I have experimented with both R-3259 and R-3259-1 carburetors on CSX2310, both tuned to the application. In city traffic driving I could not tell a difference. The main functionality change between the R-3259 and R-3259-1 involved delaying the initial opening of the vacuum secondaries, have them open faster once they started to open, and then have the secondaries close quicker in response to backing off throttle.

Disclaimer of sorts, other than the COBRA kit induction system and SHELBY BLUEPRINTED distributor a former owner had his local Shelby/Ford dealer install circa 1968-69 my old car is pretty much stock specification so I doubt that the secondaries have ever opened all the way if they ever opened. A 715 cfm carburetor on a near stock HP289 is just way too much flow capacity, 500 cfm might be stretching the how much is too much limit. I have worked on lots of vacuum secondary carburetors for people with near stock performance cars and most show signs of never having the secondaries move in street use. Race cars is another subject.

Dan