While Jon's idea of rich running during the "off-idle" transition might be theoretically correct, I've never been able to measure that situation, even when using some pretty sophisticated test equipment that cost the bean counters in the public education system a truckload of money. The transition area between idle and high speed mixture is less than 1/8 inch of movement of the throttle plates past the off-idle ports, and the transition happens so fast that there's little, if any time for that richness to happen. If it does, it's exactly what's needed to prevent a lean stumble, along with a properly adjusted accelerator pump. I'd really like to see some documentation of the idea that off-idle richness can be measured when using a carburetor that then instantly goes lean as soon as the venturi starts supplying the main air/fuel metering.
Jerry
