Ok so I have a 2002 Trek 4300. The rear derailer Shimano Acera 8 speed) will shift to the smallest cog with no problem but going up to the biggest cog it hangs up and i have to double shift to get it through the middle gears. I replaced the chain since it was bagged out and adjusted the derailer (properly I hope) according to the instructions I found online. I might need a new cog set, the teeth are pretty hooked and will be replacing that soon but are there any other possible reasons I am over looking? Thanks alot for any help.

Have you tried to "shift" the der without the chain attached? If the der movement, SIS wise, is good then each shift will have the upper pulley directly under a cog. If not then the problem has a shifter/der/cable aspect. Cable attachment at the anchor bolt or cable/casing friction are common problems.

If the der pulley matches up with each cog as you shift through the range but you still have the shifting problems then look at the chain, cogs, upper pulley wear or twested cage/hanger. Andy.

As always, helps to know when the problem started and if you had any accidents or equipment changes at that time. I would think that indexing is OK if the gears align well when shifting to small cogs, and there's nothing about worn cogs that should require a double shift. It's working on spring tension in that direction, cable pull in the other. That would lead me to check for a problem with your housing - perhaps a cable stop not working properly, too much flex, etc.

I will try shifting the derailer with no chain as soon as I get a chance. Now the chain is skipping on every cog when I pedal. No accidents,drops or mishaps to speak of to cause any of this. The shifting problem has been intermitent over the past couple years and just recently gotten this bad. It is and acera derailer and although they are opinions it seems like alot of people that have this derailer experience this type of problem and dump them as soon as possible. Any thoughts on the derailer it self?

I would check for a bent hanger.

Is there a tool or way to check for a bent hanger?

A new chain with a worn cassette can't be helping any. Could be a contributing factor.

Yeah I'm getting a new cassette as soon as possible.

OK talked to the LBS and he explained how the derailer should be adjusted( which I had done correctly) then shift to the smallest gear and down shift to next gear and adjust the barrel adjuster out until it jumps right to it. Did what he said and now the bike shifts pefectly. Upon explaining how some of the cogs look he agrees I need a new cassette since it will still skip on those gears when I really push hard. Figures something so small gives such grief lol. Thanks for all the help though I do appreciate it.