Jumping chain

[Fiona N]

I did a thorough clean of the mtb last week, including the chain which is still in fine fettle (only done a 1000km or so over winter).
But now I have an odd phenomenon - although the chain seems to be located perfectly on the rear sprockets and changes smoothe as you like, if I go over a sudden bump, the chain will jump to the next smaller sprocket. This only happens when the chains on the smaller 4 or 5 sprockets (of a 9-speed)
I've never had this before: tweaking the derailleur just makes the gear changes less good, the chain length is text book, so I'm rather stumped.

Any suggestions?

 

[Happiness Stan]

Screw the tension screw out to bring the jockey wheels closer to the cogs.

 

[Fiona N]

Thanks Stan.
Sadly I tried that and if I get the jockey wheel significantly closer to the small sprockets, it's too close on the big ones

 

[PatrickPending]

check for stiff links, is your derailleur bent tht can cause problems though it might just need a bit of tls and some oil, sticky cables - are they moving freely - can change these, new chain on old casette can cause this but it sounds not in your case - your chain could be worn though mine tend to last around 3000miles and i try and replace them to ensure long drivetrain life.

Funnily enough I've hed this problem this week, though I'm suspecting a worn casette - replaced this just need to do a test ride.

ps changing casette worked for me - it jumped a bit on the old chain but jumped a lot on the new. Now fine confirmed by test ride.

 

[Fiona N]

I'm going with the undetected stiff link at the moment - I usually check every link when the chain's hanging up drying after application of lube but I got distracted last week and put the chain back without checking. I'd forgotten this and, sod's law isn't it, that the one time you forget to do something, you get a problem