My Dirt Bike chain keeps falling off?

I just bought a Yamaha TTR 125 dirt bike used. The chain keeps coming off. Whenever I go up a steep hill, through mud, or water, it comes off. Then , just the last time I rode it, it kept coming off as soon as I started moving. Do i need to pull the back wheel back, or buy a new chain?
What kind of chain is best, and where are they sold online for a good price?
Also, does anyone know where I can buy a repair manual for it for a decent price?
Thanks.

sounds like its just loose and maybe a little crooked .. check the rollers the the chain rides aginst, if there are any
make sure your wheel is centerd and all of the spacers are on the axle and on the right sides
check the sprocket to make sure it isnt bent
look at a picture of a new sprocket online and compare to yours if the teeth look bent or worn to a noticeable difference on yours its time to replace it

loosen up the axel and tighten the chain tensioners equaly untill the chain is tight but still has about an inch or so of play up and down MAKE SURE IT HAS THE TENSIONERS!! otherwise it will just keep coming loose but while the chain is still loose hold the chain .. with a link in each hand .. try to flex the chain from side to side if there is alot of play between the links your chain is worn and stretched and it should be replaced
you can usualy find used manuals and new chains/sprockets on ebay for pretty cheap

been there done that, had the same problem.
It depends on 2 things:
1. if the chain isn't tight enough it will fall off; most likley by going off to the side.

2. if it so loose that it is actually skipping on the sproket. In this case it may have rounded the sprocket teeth. in this case you would need both sprockets and a chain

Either way the solution is to use the adjuster on the axle block to move the wheel back. but keep in mind that you don't want to make it to tight or when you go off a jump it will snap.

If you aren't sure wich one it is take it to a dealer and ask them which one it is but DON'T have them fix it or they will rip you off. It is just as easy to do it yourself with a wrench.

adjust your chain

adjust your chain. i have a ttr 125 all aftermarket. the stock chains will wear and stretch eay. even if you pull the wheel back you will have to tighten it 1 or2 times every 2 weeks

Assuming the chain and sprockets are in good condition, about halfway between the front and rear sprocket the bottom part of the chain should move vertically between 25-40mm when standing straight with no-one sitting on the bike, if it is more than that it should be adjusted until the slack is correct. If there is significant lateral movement or the adjusters are at the limit or any teeth are damaged on the sprockets both the sprockets and the chain should be replaced.

The marks on the eccentric adjusters are guides only and shouldn't be relied on as totally accurate. to make sure both the wheels are in the same direction use two parallel planks either side of the bike.

Try to adj first then be sure its running true"straight" if an extended swingarm also you it may need a chain guide some come with one , there are chain guards that mount on the rear sprocket that guide the chain also

check bikebandit.com

sell it b4 you get hurt

you would be wise to get new sprockets and a chain... be sure that all the spacers are there...

try pulling the tire back before you spend money on a new chain