[ted]

KobraKarl is right on the color of the springs. The factory 2X4 carbs came with the "long" yellow. There is a "short" yellow also available that will open earlier. If memory serves me correctly, the white spring is as light as they get.

The reason for the extremely light springs in the two four carbs is due to the vacuum signal to each carb being reduced by half thereby necessitating a lighter spring to get the secondaries to open at the correct rpm. Whenever a second carb is added, air flow that was normally available for one carb is being split between the two carbs making the air flow basically cut in half of what it was for each carb.

There are some other tricks that can be performed on the secondary opening circuit to make the secondaries more responsive or if you've put the lightest springs in and the secondaries are still opening at too high an rpm.