8 speed shifter with 7 speed cassette I just got a bike with broken hanger and derailleur. As the owner didn't want to put money in good parts, I installed a cheap Sunrun derailleur compatible with index, and a new hanger. Just after I finished the work, adjusting the cable I noticed, that it have 8 speed shifter and 7 speed cassette. I know, that it's not an issue, and there is no problem to adjust it normally and I left the 8 as ghost, but this time I couldn't adjust it: if all the gears shifting up good in 7 > 1 direction, they will not shift good down (1 > 7) and vise versa. There is a point that both directions not shifting well, like remain at gear 7 while shifter showing 6 and remain at 1 while shifter showing 2.
Am I missing something? I cleaned and oiled the shifter, cleaned outers, put a new cable and new derailleur.
Assuming the shifter and derailleur have compatible cable pulls (which you should check), you won't be able to perfectly adjust the shifter to shift smoothly on a cassette of 7, since the cable pull which moves the shifter for 8 speeds is likely not enough for 7 speeds. Thus, generally, you need to match the shifters with the derailleur and the cassette spacing. 

Sheldon Brown has some comments on 7 speed setups and running an 8 speed shifter with 7 speeds through some alternate cable routing, but given that you can buy an 8 speed cassette for like 10 bucks, it seems like an unnecessary hassle (given the extra time you'll be spending tweaking adjustment). Or you can bolt on a 5 buck friction shifter and save yourself some time too.