What will happen if i am going to replace the stock sprockets to 15/48? is it possible?

are the chain and sprockets worn out?

1st. - NEVER change sprockets without changing the chain..and vice-versa!! - they need to be replaces as a set...they wear our almost equally.. ie - a worn out chain will wear out new sprockets... A new chain will get stretched out by worn sprockets real quick!
Its east to tell what you have...

• OEM sprocket size: 16/43 - 2.68 ratio.

1. Count the teeth on your front sprocket.
2. Count the teeth on your rear sprocket.
Compare yours with stock(OEM)..
Now, if your sprockets match the stock sizes - count the number of links in your chain...(count the inner links).
If the sprockets are different sizes you adjust the chain by that number using inner links. - Usually people go smaller on the front or a larger rear.. this increases the ratio to give a quicker take-off - (more acceleration).
15/43 = 2.86
15/44 = 2.93
15/45 = 3.00
15/46 = 3.06
..when you get a new chain... better to get one a couple links too long than one link too short!!
Your local dealer will be more than helpful!!

The simplest way is to change the stock front sprocket for a larger one. Assuming the stock sprocket has 12 teeth, go to a 13 or 14 tooth sprocket. This will give a higher top end speed but will slow the bike on acceleration off the line. Simple and low cost.

Just remove the chain cover then bend the tab on the lockwasher away from the nut so you can then remove the nut. Install the new sprocket and put the assembly back together. Stock gearing is a 13 tooth front sprocket and 48 tooth rear sprocket. The stock chain is 112 links plus master link. The chain is DID 520D 112L. Please rate my answer. Thanks.

The stock rear sprocket is 41 tooth. The stock front sprocket is 15 tooth. The stock chain is 630 x 96. You can get OEM parts at www.babbittsonline.com/parts/viewbybrand/parts.aspx

Rear sprocket is $52.91. front sprocket is $21.74 and chain is $201.07
Please rate my answer. Thanks.

You didn't say that the bike was not reaching full revs. Therefor The only thing that limits top end is the gearing. Call the dealer and find out how many teeth are on the stock front sprocket and the stock rear sprocket. Chances are that you have maybe 1 tooth less on the front sprocket and 4+ teeth more than stock on the rear sprocket. The smaller the front sprocket and the larger the rear sprocket, the lower the top speed will be.

Please rate this answer. Thanks!