I had some issues with my rear derailleur and shifter so i decided to just replace them (along with the cable and housing), the gears had started to harshly skip when pedaling (didn't have the issue for 3/4 months after i converted the bike to a 1x). The shifter and rear derailleur were original (Specialized Sirrus) so figured it was time. While harsh skipping isnt really an issue anymore i cant seem to correctly index my new rear derailleur and shifter. For some reason as i go into lower gears and try to switch from gear 4 to 3 it skips gear 3 and goes directly to 2. Once it is in gear two there is not any issue switching into gear 1 (but obviously the shifter reads gear 2), but it doesnt seem to have enough tension to make another click to put the shifter into gear 1. The obvious solution would be to tighten the barrel adjuster but even when it is at its tightest (ie. just tight enough so it actually switches from gear 8 to 7) it will still skip gear 3 and go right to 2. This obviously screws up all the shifting when i'm going back up from the low gears and it will randomly switch gears as i am pedaling.

I included a video that maybe explains the issue better than i could. I've considered chain length but that seems to be adequate. Maybe i just set up the limit screws incorrectly? i am a bit of a loss here. If i should bring it to a bike shop i'm fine with that, i just like to be able to fix this stuff myself (with all of your help of course)

Shifter is - Shimano EF510 8-Speed Right Brake/Shift Lever

Rear Derailleur is - Shimano Alivio RD M-410 7/8 speed long cage

VIDEO OF SHIFTING -

Are you sure you have the shift cable on the proper side of the anchor bolt? It looks like you don't and this would make the indexing off after a few shifts. Not related to the indexing but might be for the original skipping- how worn are your chain and rear cogs? Andy

+10 Cable attached wrong, start there.

I very well could not have it on the proper side, i will fix that tonight and see

Chain and rear cogs should have a max of 420 miles on them, which doesnt seem like a lot

Anchoring the cable on the wrong side of the bolt caused me to use up a bunch of hours trying to fix it. Maddening! It sort of works on part of the cassette at a time, just enough to keep trying all kinds of adjustments. Then I saw the correct cable placement on a new bike at the LBS.

Note that every reply went right to this cause! Has this happened to lots of mechanics?

It affects it so much because the mounting position changes the lever arm length, thus the indexing no longer moves the correct amount per shift.