Front derailleur twist shifter replacement - from small to big ring in one click Replaced front OEM Grip Shift SH-F3 barrel shifter with Twist Grip M25-7 shifter on Vintage consumer-type bike w Shimano GS100 and 3 chainrings (x7 cassette).
Mounting went fine, cable run is clean and clear. Backed off barrel a few turns and with shifter on “1” and slack taken out connected the cable to the derailleur.  Minimal adjustment needed to the L limit.  But the shift from “1” to “2” skips the middle chainring completely and takes the cage all the way to the largest, and there’s no apparent index adjustment - nor instructions or even a product listing on the website (it was an amazon purchase of a “Shimano-compatible” - please limit the chastising). I’ve reached out to MicroShift for support but no response.
I do find it interesting that the new shifter 1-2-3 markings are not equidistant - there is greater spacing between 1-2 than 2-3 which I don’t get. For what it’s worth no amazon comments similar to my problem.
In my understanding Shimano have massive market share and that the mass-consumer bike market is highly standardized in components, altogether suggesting I’ve done something wrong - that’s where you come in - even though the mechanics of these things aren’t rocket science.
Any advice would be appreciated.
My first thought is that the new shifter isn't compatible with the chainring spacing, but unless this group has a super-narrow profile I can't think of one that would be as wide as a typical first-to-third chainring spacing.
In my experience, the markings on twist-type shifters are not always equidistant, and may be due to non-linear throw of the derailleur or non-linear take-up of cable in the shifter. Check that the cable is properly routed through the shifter as well as to the derailleur pinch bolt. I have seen the latter cause this issue.
If you have an alternate derailleur (top swing vs. bottom swing) you can try that if space allows.