I once visited a guy who had recently bought a used dirt bike from someone who bragged about being a motocross champion. My friend proudly showed me his new-to-him dirt bike and pointed out the “racing” sprocket on the rear tire. The seller of the dirt bike told him the hook-shaped sprocket teeth grabbed the chain better for racing.
The seller may or may not have been a racer, but he was definitely dishonest. The sprocket was completely worn out (creating the curved teeth), and the chain along with it.
Fortunately, replacing a dirt bike's sprockets (it pays to replace the front and back right away) is relatively straight-forward and inexpensive.
Most dirt bike repairs are easier to do if the bike is up off the ground, and this one is no exception. Get it up on a stand (or blocks of wood, or a pile of dead cats, or what have you) so that rear wheel is off the ground.
Undo the rear axle nut. This can take a lot of effort and strain – it's best if someone holds your dirt bike so that it doesn't tip and leak gas... and no one should stand behind you in case you strain too hard and leak gas.
Once the rear axle nut is off, loosen off the rear wheel tensioners, or adjusters – those nuts you turn to tighten or loosen your chain. Loosening these now will give you a little more play and wiggling room for the next step.
Remove the rear axle. Watch for other parts that may want to fall off and hit the floor, especially those associated with the brakes.
Remove the rear wheel. Watch for even more parts that may try and leap to their death. Slip the chain off the wheel.
At this point, I like to set the dirt bike's rear wheel on a pail or crate with the sprocket facing up and the brake disc facing down – protected from the accidental clumsy-ninja-with-nunchuks impression I inevitably end up doing while wielding wrenches.
Undo the bolts holding the sprocket to the wheel and remove it. The old sprocket is generally useless so you can scrap it – or give it to one of the neighborhood kids who will then think you are really cool. This can buy you some goodwill with the neighbors the next time you tear down the street to test your latest repairs.
Clean away any dirt or grit that might have built up on the rear wheel and bolt the new sprocket in place. This may seem obvious but, just in case, if your dirt bike has a dish-shaped sprocket, make sure you put it on the same way the original one was facing. Thread locking compound (such as Loctite) is a good idea for the sprocket bolts BUT place a little under the countersunk head to help prevent them from spinning. If the nuts are self-locking or have lock washers, thread lock won't be needed on the threads.
TIP: For the couple of extra bucks it'll cost, it's worth using new bolts for the rear sprocket.
Now move on to the front sprocket.
Remove the front chain guard and whatever is holding your front sprocket onto your dirt bike (the sprocket keeper). This will usually be a large nut and lock washer, an E-clip, or a clip that's bolted to the sprocket
If you bought your bike used and it has an E-clip, chances are someone has replaced the original nut and washer. You can check with your local dirt bike parts shop – give 'em the model and year and they can tell you what the factory part is/was – or just take a close look at the front sprocket assembly. The lock washer will fit over teeth that are machined into the counter shaft to keep it from turning. Then, once the nut is in place, a part of the washer gets bent up against one of the flats on the nut to keep the nut from turning. If you have an E-clip, just pull it off... if you have bolts, lefty loosey.
Installing the sprocket is just the reverse of what you did above.
It's usually a good idea to change the chain when you change sprockets. Your call. Slip the chain over the front sprocket, then over the rear sprocket. Slide the rear wheel in place and slide in the axle. Put the axle nut on loosely. Snug up the tensioners or adjusters, and then tighten down the axle nut.
Remember to check your chain tension at short intervals for the first ride. And, after that, well you're already checking your dirt bike's chain before every ride... right?