phantom shifting, with a difference
On my fairly old road bike, the drivetrain has developed a strange fault: Whenever I shift down onto the inner chainring, the chain shifts UP one sprocket at the back. When I shift back to the big ring, it shifts down again. Also, when it's on the inner chainring, the chain makes a lot of noise from the rear sprockets, even though it's fine when it's on the outer chainring. The drivetrain is made by a company called "Gippeme", and I'm told the parts look very similar to Campagnolo parts from the early 1980s, so I'd imagine that's when they're from. The rear shifter is an indexed 7-speed, but the front is friction-only. I'm not running the chain at too great an angle, so what's going on? More to the point, how do I stop it?
Thanks in advance
Changing from one chainwheel to the other changes the orientation of the rear derailleur pulleys and to some degree the rear derailleur body. If the dropout is not aligned properly or even worse if the derailleur/pulley assembly has been twisted that can easily result in enough movement of the top pulley to shift to a different gear. I would bet your rear derailleur took a wack in a fall or while parked/transported.