If you’re rapid fire trigger shifter is having trouble catching or shifting properly, there is an easy way to get the shifter working properly again.
There first thing to do is rotate the brake lever, which involves loosening the clamp holding the shifter to the handlebars. This will give you better access to areas of the shifter.
You will need to have access to the hole that leads into the shifter. This area can often get clogged up with grease, thereby making it stuck and unable to shift properly. Use WD-40 and spray this into the hole to break up all the grease, old dirt, and oil. Get a lot of it in there, and then begin to work the gear shifter, turning the cranks to allow you to cover the whole range of your gears. The WD-40 should get things working again, but only for the time being. WD-40 isn’t a lubricant, it is a solvent, so it will only work temporarily as a lubricant.
Now, with the grease broken up from the WD-40, use an actual lubricant and spray this liberally in the same hole, again working the gears through. Now clean off as much excess dirt and grease as you can, move the shifter lever back to its original position, and it is good to go.