Derailleur or pulley?

[skyemc85]

Sorry if this has already been covered, I'm new to all this

I have recently had my cogs and chain replaced on my mountain bike due to wearing which caused my chain to slip in a number of gears, have spent a few days adjusting the derailleur thanks to online videos, and seem to have everything working perfectly with the exception on one gear,

when the chain is on the largest chainring and the largest on the back the chain seems to jump and make a loud noise, I have noticed the lower jockey wheel may be the cause of this, as this is where the chain seems to be jumping from and not on the actual cassette, but this only happens in this gear, it can be tricky to come out of the gear but as soon as I am out of it the gears work perfectly again. I have been told I need a new derailleur, but if that was the case would this not effect more than one gear?

any advice would be appreciated, thanks

[nfmisso]

big - big is not a good one to use on most MTBs - which usually are spec'd with wider BB, pushing the big chain ring further to the right.

[1FJEF]

What he's saying is that when the chain is on the front middle ring, you have access to most of the rear gears, when it's on the right big front ring you have access to all of the rear gears except the 2 or 3 biggest ones. When you are on the little front ring you only have access to the the biggest gears on the rear.
On most bikes, you run into "cross chaining", just as you are when the chain is on the right front ring, and the left (big) rear ring.
Looking from the top, see how far the chain is trying to flex from the outside at the front, to the inside at the rear? That's cross chaining.

[skyemc85]

Thanks for the replys, so I'm guessing that it should be fine to ride and I don't need to have a new derailleur fitted?