Aggressive gear changing problem

[maxfox44]

Hi Guys

Using this 26yo MTB for commuting. Been fine, with the occasional adjusting of cables etc.
Recently, when I change gear the chain ends up on the right cog, but the action is really aggressive (only way to describe it). Before it would slide on and off, with minimal noise and effort.
The limit screws seem in the right position, cable is fine and slides okay, the paddle shifters are indexed okay. All a bit of a puzzle. The reason for posting now, is because twice the chain has jumped off the cassette and become stuck. It is easily freed, but slightly worrying in the middle of traffic, when it does this.

Any pointers would be most gratefully received.

 

[Tim Hall]

Is the chain 26 years old too? And the chain rings?

 

[maxfox44]

Yes, but hardly done any miles until the last 4 months. The chain looks good and measures as no wear/stretch.

 

[adamhearn]

If it's occasional, perhaps stiff link(s) in the chain? Rear wheel off the ground, set gearing to smallest rear cog,pedal slowly and examine the chain carefully and see if any link jumps up more than it ought to. If you see some then you can try soaking in oil, working the joint or similar. Extreme cases may benefit from some mechanical adjustment. New chains are cheap...

If it's all the time then it can only be indexing (assuming the chain isn't a rusty wreck which you suggest it isn't) or an outside possibility of a sticky cable?

 

[maxfox44]

Thanks @adamhearn I suspect the problem might be something to do with the b-screw I think it's called. The screw which moves the mech closer/away to the cassette.