I know this question has been asked before but I want to make a thread to say hello as well. THis is my first time here and so far I like it .
From My signature you can see what I have. THis bike was given to me and I have been out of the loop for way to long. My back Derailer seems to have a problem. It seems to be skipping a gear going in either direction. Is this normal? I havnt done to much work on 21 speed so I wanted to make sure
THanks
Jason

First get a new gearshift cable it is probably gummed up, and clean the rear derailler cage. If that doesnt work it might be due to a worn chain, which would also require new sprockets.

look up https://www.sheldonbrown.com/articles.html

and https://www.parktool.com/repair_help/FAQindex.shtml

Most likely you need to adjust the derailluer. Try adjusting the barrel adjustment knob attached to the derailluer. Look at the chain from the cassette to the derailluer. It should line up.

THe cable is doable, the chain is doable but the sprockets seem a little questionable. Couldn't I just clean them out real good? I mean to say. Take it completely apart clean them with auto cleaner or what not and the regrease them were need be and go from there?

I am not familiar with the Trek lineup. Do you have GripShift? If you do, changing the cable and readjustment should take care of the problem(Get a 1.1mm cable if you can, it really helps)

If you have Shimano shifters, make 100% sure your der. hanger and the der. itself are not bent. The hanger can be straightened (Or replaced if the replaceable type) The der will need to be replaced if twisted. Altho if you feel confident of your bendy skills you can tweek it a bit to try and get it straight. Good luck.

Yes they are grippers, so I have to go a click here or there to get my desired shift. Will have to take a look at that though THanks

Those shifters are the problem. They used a pretty stiff cable and it is looped 360 in the shifter, creating its own little spring. The lighter 1.1mm cable has less spring and less drag in the housings. The difference in the crispness of the shifting is amazing

ummmm, Not sure I am fully understanding. You say that the shifters are the problems but then you talk about 1.1mm cable? So should I replace the cable or invest in new shifters?

The design of the shifters causes some extra stiction and spring in the shift cable. They are the cause of a lot of problems like the one you have. However just replacing the cable (Less $$ than a shifter) will take care of the problem for quite a while. Switching to at least the stainless 1.2mm cable(Everybody has these) makes a big difference. There is no need to change the shifters unless you get frustrated running the cable (A minor pain on the right shifter)