What makes a drivetrain good? In order of my priorities: Smooth, reliable shifting. Light weight. Quiet running. Low maintenance. If you bought a used bike with a mixed component set you'll have to live with the mistakes of the one who put it together or figure out what's wrong. It could be as simple as a mis-aligned rear derailleur. You'll have to learn the basic things to look for, like the derailleur and chainline alignment, correct chain type and length, condition of sprocket teeth, freewheel/cassette bearing adjustment, etc.