So I bought a set of Suntour derailleurs for the bike I'm working on and I'm having some issues getting the RD just right. It seems to be functioning ok (a bit of an issue when I go large cog/large chain ring but no biggie), but when I peddal backwards the cage tends to pivot with the lower pulley moving towards the chainring before the chain begins to rotate. This causes the chaing to become really loose on the top and drag across the chain stay.

The pulleys seems to be spinning fine and I've tried adjusting the angle of RD, but nothing seems to work and I'm really at a loss here. When I hold the lower pulley as I spin backwards - no problem. So I tried tightening the screw that the cage pivots on, but it is as tight as it goes. Any othe suggestions?

I know the easy answer is, "don't peddal backwards" but the bike isn't for me and I'd rather not hand it over with a list of instructions.