Gear shift problem from 7th to 8th rear sprockets

[girish.kale]

Hello Folks,
My name is Girish and I recently bought Trek 4300.
I experience below issue with the bike. Maybe something very simple to correct, but I’m a naive to this.
Here is what I face:
When I shift from 7th to 8th number, the chain does not immediately jump on to the smallest sprocket (8th gear). However it moves only if I paddle very light and if otherwise, I hear a chain sound with the sprocket.
I tried adjusting the cable tension. If I add more tension to the cable (moving the cable tension adjuster anti-clockwise) then when I shift from 8th to 7th sprocket, the chain actually jumps onto the 6th sprocket, and if I reduce cable tension (moving the cable tension adjuster clockwise) it shifts correct from 8th to 7th. However, in either cases, if I shift from 7th to 8th, the chain does not jump swiftly onto the smallest sprocket.
Please can you help?
Many thanks,
Girish

[JonB]

Try turning the high adjustment screw (should be marked with an H) anti-clockwise (i.e. letting the derailleur move further away from the wheel. This should work (I had a similar problem not so long ago)

[DaveM]

It does sound like a problem with the cable tension. If you cannot find a good balance where you can't get it to shift right in both directions, it may be a cable problem or derailleur. Next thing I would try is taking the cable out, re-greasing it, and checking all the cable housing to make sure there are no kinks or splits. The cables need to have nice smooth bends and if there is a sharp turn anywhere it can mess up the shifting.
It could also be caused by a slight bend in the derailleur. If you look from behind the bike, the two pulley wheels should be exactly parallel to the cogs in the back and sit directly in line with the one of the gear that the bike is in. If anything looks like it is not perfectly in line, you might need to get some help to get the alignment fixed.