Older RD having issues downshifting
 

I have an older Suntour 7 RD that won’t change to the smaller cogs. It shifts up fine, but shifting back down it “gets stuck” on the middle cog (5 speed freewheel). I tried adjusting the H/L screws, but that doesn’t appear to be my issue. I thought maybe the spring was shot, but it seems pretty lively when I remove it. Any help or insight would be appreciated.

FYI - This is a replacement RD for the stock Simplex that came on a '72 Raleigh Grand Sports that I restored a few months ago.

Sounds like the cable needs to be cleaned/lubed.

I agree with Bill , but also check the B strew on the derailer if it have one . The derailer's pulley might be hitting the cog . The derailer's hanger can also be bent . the hanger on the frame or on the derailer itself.

I had a recent issue like this as well. It turned out to be a cable housing issue just as Bill talked about. I removed the RD and soaked it in solvent and dried and lubed it. But still getting stuck. Looking at the cable housing I found a pinched end underneath the cable housing caps on two of the cuts, and the plastic guide under the BB was grooved like I've never seen. But luckily it was a fairly standard part I have in stock and easily unscrewed it and replaced it. After fixing those issues and replacing the fat, stock Shimano braided cable with a cheap, thinner $0.50 cable, the shifting works great now. I think the issue was also, non-shimano housing with shimano OEM thick cable.

Thanks I'll check, but I replaced the cables and housing during the restoration.

It sounds like there is excessive friction somewhere in the system. When you replaced the cable housings did you cut the ends square and file them flat, then install the correct ferrules? Is the cable guide under the BB in good condition with the cable correctly routed? Did you replace the small loop of cable housing which goes into the derailleur and is it correctly sized so that it makes a smooth curve with no kinks? Are you sure that the shifter is paying out cable smoothly? Fiddling with the limit screws will do no good if the derailleur does not move freely across the cogs.