Bicycle Mechanics - Shimano Acera, can't get to the highest gear.

I have a 3700 2009 Trek MTB and am having trouble getting it to the highest gear.
The gear says Shimano Acera, and it has click style shifters. I can't have the gear on the left at "3" and then get the one on the right to "7". The gear says 7, but the chain doesn't swap over. If the left gear is at "2", then the right can change over to 7. I can then pop the left side into "3", but either the chain keeps slipping over and eventually shifts, or it shifts right away and rattles.

What is the adjustment?

It might be the limit screw being too tight. Either that or your cable is too loose.

I'm betting that the cable is too slack. If it worked okay when you first bought it, then the cable has either slipped or has stretched just a little... that's normal. The cable is easy to adjust... once you've seen it done.

I'm going to suggest that you take it to the shop, and ask if you can watch as the wrench fixes it. You'll be good to go in no time.

Two unsolicited tips:

Buy a set of hex keys. Figure out which ones fit the derailler cable pinch bolts, the brake, stem, and seat bolts, and take them with you when you ride. Stuff works loose, and a cheapo hex key can keep stuff from failing... or falling off.

If a shifter suddenly gets weird... It hangs up between gears, is extremely sluggish, or all of a sudden doesn't want to index on the right gear... or maybe indexes between gears... You have a cable that is just about ready to break. Typically, it'll act weird for a couple of rides before it snaps, so you'll have ample warning. If you even suspect a failing cable, remove and inspect it as soon as you can... It'll be frayed, with broken strands, at either the shifter end or the derailler pinch bolt. Don't ignore it. If it breaks out on the road or trail, you'll have to McGuyver a repair that'll get you home.

It might be the limit screw being too tight. Either that or your cable is too loose.

One of the cables are very loose. When changing gears, doesn't the shifter tighten or loosen it slightly? If I tighten it, how much tighter would throw of the system?

If the cable's slack, shift all the way to the gear that it doesn't jump into, then wind the barrel adjuster out counter clockwise (it's the black plastic bit that screws into the back of the derailler) a half turn. Pedal, if it doesn't shift up, wind it counter clockwise another half turn. If you keep winding it out and it isn't shifting up, then the limit screw is the problem.

Hope that makes sense.

I found the screw and adjusted it. I wish I read Tywin's post before making the adjustment because I made a few turns rather than 1/2 turns.

If the cable's slack, shift all the way to the gear that it doesn't jump into, then wind the barrel adjuster out counter clockwise (it's the black plastic bit that screws into the back of the derailler) a half turn. Pedal, if it doesn't shift up, wind it counter clockwise another half turn. If you keep winding it out and it isn't shifting up, then the limit screw is the problem.

Hope that makes sense.

OK, I had a chance to get it on the road and make adjustments.
I can make it get into gear 7, but at the expense of loosing gear 1.
When doing a whole lot of 1/4 turn adjustments it slips alot, and some clicks share a gear. Like click 1+ click 2 is still in gear 1.

Is there some other part that can get adjusted? Or is it a fine patient adjustment with a sweet spot?

Fine patient adjustment with a sweetspot. Best thing to do is follow this site:

http://www.parktool.com/repair/readhowto.asp?id=64

Do everything in order, and you'll be fine. It might take some fiddling at first, but with some practice you'll get it.

I had the same problem with my Trek 820 with a Shimano Acera rear derailleur and grip shift. It would not shift to the outermost cog or gear 7. I shifted to the gear it wouldn't shift to and turned the black plastic Tywin mentioned until it looked like the chain was aligned (a few counter clockwise turns). I rode it around but then it wouldn't shift to the innermost cog, so I started doing the 1/2 turns the opposite way until it shifted to the innermost cog. Finally, It wasn't shifting and was clicking from gear 4 to gear 5...I shifted to 5 and then turned the black plastic 1/4 turns until it shifted.