What is wrong with my bike's gear shifting?

[Marlogism]

my brand new mountain bike's shifting is very bad. the chain keeps moving between gears by its own and it doesnt shift right when i want it to. i must keep holding on the shift down hand a bit so that the chain stay in place.
i thought messing a bit with the limit screws would solve it but i think i have over done it and made it worse.
what should i do?

[cny-man]

Limit screws do not have any effect on shifting between gears. A brand new bike will generally have free adjustments available for 30+ days - best to take advantage rather than risk messing up and maybe voiding the warranty. If you bought unassembled then consider a bike co-op for help if there's one nearby.

[Old Retired Petty Officer]

I have a 6 year old Next 18 speed mountain bike and it is not shifting at the rear very well and the front only works on two sprockets. It had been sitting for a considerable time outside. I rode it three days per week for about 8 to 10 miles for exercise. I had to revert to walking because of it.
It the cost of repair exceeds what I paid for it, I plan to let it go to charity.

[cny-man]

(10-13-2014, 05:34 PM)Old Retired Petty Officer Wrote:  I have a 6 year old Next 18 speed mountain bike and it is not shifting at the rear very well and the front only works on two sprockets. It had been sitting for a considerable time outside. I rode it three days per week for about 8 to 10 miles for exercise. I had to revert to walking because of it.
It the cost of repair exceeds what I paid for it, I plan to let it go to charity.

If the levers move the derailleurs it's often just a matter of adjusting the cable and perhaps some lubrication. Bike co-op can help if you have one nearby, videos on this site are supposed to be pretty good, but I suggest you look at written instructions on a good site first. Google "adjust (front/rear) derailleur" - sheldonbrown.com and parktool.com sites are recognized as being the best.

[Old Retired Petty Officer]

(10-13-2014, 07:06 PM)cny-man Wrote:  

(10-13-2014, 05:34 PM)Old Retired Petty Officer Wrote:  I have a 6 year old Next 18 speed mountain bike and it is not shifting at the rear very well and the front only works on two sprockets. It had been sitting for a considerable time outside. I rode it three days per week for about 8 to 10 miles for exercise. I had to revert to walking because of it.
It the cost of repair exceeds what I paid for it, I plan to let it go to charity.

If the levers move the derailleurs it's often just a matter of adjusting the cable and perhaps some lubrication. Bike co-op can help if you have one nearby, videos on this site are supposed to be pretty good, but I suggest you look at written instructions on a good site first. Google "adjust (front/rear) derailleur" - sheldonbrown.com and parktool.com sites are recognized as being the best.

Thanks for the info cny-man!