When I shift to the larger cog in the back, the derailleur pulls the chain back to the small cog automatically. I thought this was due to too much tension in the shifter cables, but that doesn't appear to be the case...

Haven't fiddled with the derailleur set up yet, the adjusting screws are on the side of the derailleur. I believe it's old Suntour stuff.

Thoughts?

Thanks!