differences in 28mm

Saw some carbs for a Z1/KZ advertised as "28mm S-2 not plain 28mm". whats the difference between "plain" and "S-2" ? thanks
Scott

No idea what this advertisement was talking about. Sorry

Is this them ? Ebay #4620259779
They are the second style of VM 28 that were used on the Z-1. The earlier carbs are more desirable because they have a removable threaded plug on the bottom for easier jet changes.
Either carb is good and the better ones to have are the earlier ones. They are easier to make jet changes.
The S-2 carbs have a different slide cut out that gets rid of the off idle stumble common to the early Z-1s. The earlier carbs can be modified to have the same cut out on the slide just by shaving .030 off the bottom of the slides. This changes the cut out to have a slightly richer mixture right off of idle.
Fish

A larger slide cut out will lean the idle mixture, not richen it. It allows more air through the slide as it is opened through the entire range of operation but doesn't matter much when the slide is all the way open. The early (73/74) Z1/Z1A had 2.5 slide cutaways and required a correspondingly larger pilot jet (#20) while the Z1B (75) version of the carburetor required only a #17.5 pilot jet as its cutaway is 1.5. While it is possible to change main jets on the early carbs without removing the carbs from the bike, they have a disadvantage while synchronizing as the #4 carburetor slide is a fixed position and the other three must be adjusted to it while the Z1B assembly allows adjustment of all four slides. Both the Z1/Z1A and Z1B carbs have relative advantages/disadvantages but from a performance standpoint, the earlier carburetors are capable of flowing more gas because of other internal characteristics that have little to do with the slide cutaway or synchronization mechanism.

When the slide valve has .030 shaved off the bottom, it reduces the slide cut out, changing the 2.5 cut out to about a 1.5 cut out. This richens the mixture right above idle and gets rid of the off idle stumble.
I have done this modification and it works every time.
Either early set of carbs are equaly a pain to sync. The adjusters are very touchy with the early external adjusters.
Fish

Just curious Fish...29mm Smoothbores...not that I'm having that problems,BUT...would that work for those carbs as well?...

Sandy,
I didnt experiment with the slides to come up with this. I have a folder full of magazine clipping for the Kaw 900's from the 1970's and this is one mod that is in it. It took care of the stumble on my 75 900 (with earlier carbs) and I have done this mod to the same carbs many times.
I would imagine that it could be done with any VM carbs, however, you would need to know what cut out you would need and how much to remove from your current slides to get the cut out that you need.
Fish