rear cassette shimano 10 speed chains skips when climbing hills

[reacher]

hi
hopefully some one can help me ,
its an ultegra group set , 12 x 25 rear cassette , when i'm climbing hills over 10 % the chain when i put it on the rear 23 or 25 cog keeps jumping back , so i put it on 23 it jumps back to the 21 same on 25 it skips back to the 23 or 21 , if i put tension on the front shifter ie hold it in as though changing gear it will stay in the 25 ,
anything around 10% or less it works fine stays in the correct gears ,
i understand the way to tension the cables just not which way , ie should i tighten or loosen the cable and is it a problem with the high low limit screws as well ?
thanks

 

[PpPete]

Not a problem with the limit screws, as that would stop you getting to the top of the cassette in the first place.
Sounds like the spring in the mech is overcoming the spring in the shifter.
Is the chain the right length?

 

[reacher]

its very possible that it could be a link at least too long , i ruined one making it too short so went on the side of caution with this one , would that possibly cause the problem ?

 

[PpPete]

its very possible that it could be a link at least too long , i ruined one making it too short so went on the side of caution with this one , would that possibly cause the problem ?

Unlikely IMO

 

[BimblingBee]

Sounds like an indexing problem, or have you recently swapped chains?

 

[reacher]

yes , new chain this summer ,
what does imo mean please

 

[BimblingBee]

In My Opinion!

Mmm, if its fine the rest of the time but just jumping under load then it does sound like they've not meshed nd something chain or cassette, is worn.

Doubt that the rear mech is overcoming the spring in shifter as its a ratchet system!

 

[reacher]

its a brand new cassette , happened on the old one as well , and a very recent chain just a couple of months old ,

 

[BimblingBee]

Mmm, even more interesting!

I'll go for indexing, try a bit more tension at this will mimic the holding shifter which you say works.

As for relating new/recent to wear, then it's all relative and would depend on mileage and othe environmental factors.

 

[boydj]

I'd tighten the cable half a turn - and another half turn if the first attempt does not fix the problem. It does sound like an indexing problem, since all the hardware is relatively new.

 

[reacher]

ok , thanks guys , tighten then , i'l try it out on the road