jumping gears

[Twistygit]

I've got an old mountain bike with 27 gears whenever I'm climbing it's okay till i get down to 13th then it jumps into granny ring but it 'll stay in 13th okay, any ideas why the only thing i can think of is the chain to tight. Do you think that could do it?

[nammynake]

Gears need indexing. Look for a barrel adjuster on the rear derailleur. Plenty of guides online - try Sheldon Brown. It's a 2 minute job.

Edit - or possibly the limit screws. Not sure what "13th" gear is...

Edited by nammynake on Saturday 5th April 19:38

[WinstonWolf]

Yup, classic indexing fault. Could also be the front mech from the description

[barker22]

Do you mean when you are in the largest gear at the back and it is jumping down to the smallest at the front? If so you generally need to use the gears that way to keep the chain straight. Apologies if this is not what you meant.