I tried to convert my bike from 3×7 to 1×8 because I trashed the rear wheel. I kept my rear derailleur (7/8 speed compatible) and bought a new shifter, new cranks and put a Sunrace 11-40 8-speed cassette on the new rear wheel. The cassette came with a derailleur hanger extension. Here are the problems I am having (counting cogs from inside to out / biggest to smallest).
1) Doesn’t want to shift into fourth cog. Will not even start to shift when downshifting from 5 to 4. Either have to give shifter a bit more pressure or go down to 3 and back to 4. I have tried adjusting the derailleur and even took the cassette off to make sure I had everything together correctly. It is as if there is a bigger space between those two cogs. Of course these are the two most useful cogs.
2) Chain skipping on 6th and 7th cogs. I was chalking this up to lack of chain tension because I can’t see anything else wrong and 8th cog works fine. Chain can’t be shorter or would rip off the derailleur in 1st cog. I am considering either a longer b-screw but leaning toward new derailleur because there was some damage at the very bottom. I will also get a new chain. Is there anything else it could be? Like not enough chain wrap?
I wish I would have dragged my feet more because the Microshift Advent group set has finally hit the market. Would have been a bit more but 1×9 with clutch and slightly more range.