I am new to the exciting mountain bike world. So far I am enjoying it a lot and can’t wait to go on a long ride. I need help adjusting my rear derailleur. I have a Marin bobcat 3 (stock). It has a Shimano Altus 8-speed match to a SunRace 8-Speed, 11-34T cassette. The derailleur was set up by the factory and after 2 rides my 4 and 5 gear jerked (jump) so I was not using them in the trail. At home, I removed the cable from the derailleur and pull on it to tense it again. All the gears shift smoot when the bike is in the rack. I took it to the trail and I still have the same problem. I adjusted the cable’s length at the shifter and now I can use gears 1-6 and 7 and 8 now jerk. What can be causing the issue? How to fix it?
Check the straightness of your derailleur hanger. The are made of a soft metal and can be bent easily (in shipping, in the garage, on the trail, even setting the bike drive side down).. It only takes a slight bend or twist to effect shifting. There are tools designed to check and adjust it, but you might be able to don’t by hand. Don’t try bending it unless you have a spare hanger handy. They can break easily. You should have a spare handy anyway
Also make sure the cable is routed in the derailleur correctly. The cable should seat in the retention clamp straight and rest in the channels in the derailleur properly (depending on the derailleur design).
Make sure the limit screws are set properly. They restrict how far it can travel and might not seem to effect shifting, but if the derailleur is not in the right position when you adjust the cable, you are starting with a slight error in position that can compound with any other issue.
The Park Tool YouTube channel has good instructional videos on just about any bike maintenance topic. It is worth checking out, especially if you are new to the sport