Chain jams on higher gears

[aga021]

Hi,

I have very little experience of mending a bike myself so I apologise if this is a stupid question.

I have just acquired a mountain bike, and changing between the lower (rear) gears works, although is a little "clunky." But if I go to the higher gears -6,7,8- the chain will jam after about 20 seconds, and the pedals will not budge forwards until I lift the chain off of the rear sprocket and replace it. When this happens, I can back pedal but with absolutely no tension. It's not so fun to sort this several times a journey, so I'd be most grateful if anybody has any advice!

(The previous owner says they did not have this problem, so it may be related to me knocking something when I replaced the rear inner tube).

Thanks!

[nfmisso]

need pictures from many different angles

[RobAR]

Is the chain coming off of the smallest cog and getting jammed between it and the frame?

[Bill]

Lemme add Rob sticking chain link . BUT listen to the above first...