Help with low gear.
I apologize first for the noobness of this post.
So I bought a new Gary Fisher Pirhana. Each time I go out it seems like when I shift into the set of low gears it doesn't engage and keeps jumping gears. The first time it happened I went to the shop and he told me he couldn't find anything wrong, but the bike with weight on it could have something to do with the problem. He tightened something up and it worked the next time I rode. Now it happened again. Where should I look? What should I tighten? Should I take the bike back (still within my 30 days).
My first guess would be that it's your derailleur cable. When new, metal derailleur cables will stretch out over time until they find a happy place. When they stretch they don't pull your derailleur as far as it needs to go to accurately shift gears, leaving the the chain partly-shifted between gears and trying to jump between cogs. If it's still warranteed, take it back and have them fix it for you, or reference either the Park Tools instructional maintenence site or pick up a book such as "Zinn and the Art of Mountain Bike Maintenence."
Park's page for derailleur adjustment: http://www.parktool.com/repair/readhowto.asp?id=64