Rear mech shifting issues: Mech or chain?

[Andy in Germany]

Currently working on a bike that works fine except for the rear gears: when the gears are changed one at a time, they index perfectly; if we try to change three or four gears at once the chain jams and you can't turn the pedals. This happens in both directions.

Is this likely to be a problem with the mech or the chain and cassette itself? the chain isn't new but doesn't appear to be worn.

 

[Ajax Bay]

Worth checking the spring tension on the RD cage is sufficient: is it wrapping effectively (compare with another by hand/feel). Deficiency here might result in this symptom.
I'd also check that the guide pulley is sufficiently clear of the sprockets. Maybe a turn of the 'B' screw might reduce this.
Can't see how the cassette or the chain would cause this.

 

[Big John]

Hi Andy. Any chance of a video clip so we can see it? It's hard to sort stuff sometimes without seeing it actually happening. I normally go through all the variables that it might be. Worn cable, worn cassette, worn chain, sticky freehub, etc. Takes a few seconds to check chain wear. You may even have parts that aren't compatible e.g. SRAM shifter/Shimano rear mech. Have you road tested it? Sometimes the bike behaves differently out of the stand.

 

[Dogtrousers]

I'd also check that the guide pulley is sufficiently clear of the sprockets. Maybe a turn of the 'B' screw might reduce this.

This was my first thought.

So it's probably wrong.

 

[Andy in Germany]

We tracked this down this morning: the mech hanger was very, very slightly off true. This didn't make any difference when we went one gear ar a time, but changing several gears at once from the smallest threw the chain into the frame and it jammed between frame and cassette. Straightening the hanger solved the issue and the bike is now on sale.

I'd have taken a picture but it was a busy day today.