Shifting problems

[John123456]

Hi, I have a relatively new decathlon bike (drop handlebars), and recently a shifting problem has come up. Whenever I try shifting up on the right (i.e. towards the smallest cog on the rear cassette), my bike shifts up several gears at once, despite me only pressing the shifter trigger once. Normally it jumps around 3-4 gears, but sometimes the chain jumps right from the largest to the smallest cog with one click, which makes it very difficult to adjust suddenly when I'm out riding because it suddenly becomes very difficult to pedal. My shifting towards the larger cog on the rear cassette is fine - I know that it's normal to be able to shift a few gears down when shifting in that direction depending on how far you push the shifter lever on the handlebars. Shifting in the other direction should, however, only shift one gear per click of the shifter. I'd be grateful for any help/advice. Thanks

 

[I like Skol]

Sounds like the shifter is goosed. From the description of the problem it doesn't appear to be a cable or indexing adjustment problem.
If the bike is under warranty I would take it back to the supplier as shifters are not cheap.

 

[Ajax Bay]

You may find, as a temporary work around while you follow @I like Skol 's advice, is to apply a little inward lateral pressure to the brake/shift down lever as you attempt to change up one sprocket. If you apply too much pressure then the shift up lever won't operate/work. You need to get a 'mummy bear' balance.

 

[Racing roadkill]

That sounds like the internals of shifter are knackered. Decathlon have a very good warranty system.

 

[Nigelnightmare]

It sounds like the pawls inside are sticking.
The factory grease dries out and gums up the internals.
Try spraying some lube into it GT85/WD40 & work it through the gears.
If that works then lube it with oil on a regular basis.
( I use chain lube on mine.)