The problem is that my bike is very slow at start. That is it has a very poor acceleration. Another problem is when I goes uphill I have to changed gear. Moreover, when it is in slope I have much speed but the problem is that I ride almost everytime on the 4th gear.
Can anyone help me so that I can get a good acceleration and a good speed too. If there are other modifications that I can do too...
Please note that the original sprocket is 15*39 sprocket.
I have a 200cc chinese bike, the actual bike should not matter for the purposes of this discussion since there are some basics to how they work which apply no matter who makes it. I have been through this with mine as well and will cover what I did that helped
When you changing the gearing you alter where the power is (more or less). You may have gotten top end speed but at the cost of low end performance.
There are some jets that you can replace in the carb, these are usually simple to replace requiring only basic tools and basic knowledge. Tweaking a carb for the exact settings is specific to that bike, that carb, and the air intake, engine and exhaust that you have specifically. You may have to try a couple different jets to get it right. This can be time intensive because you have to change 1 thing then test, then change something else and test again.
For 0-1/4 throttle you generally adjust the pilot screw.
For 1/4-3/4 throttle you generally adjust the needle
for 1/2-full throttle you generally adjust the main jet
In addition you may find that a freer flowing exhaust and possibly a freer air intake could help as well. The air intake is questionable on many bikes now because they are often tuned to lower noise and in many instances, because they are resonant with the engine you will not see big performance gains however you did a big bore kit which alters that dynamic and it could be harming performance. You may have to replace the air box or drill some holes in the air box to allow for more air and air to pass more freely into the engine.
Some countries have bans on replacing the exhaust with anything louder than what came with the bike from the factory. I have heard even here in California in a couple years they are going to ban that (the law has allegedly passed and we are in the grace period now). You may want to check local laws before changing the exhaust to a higher flow system as those are usually louder as a by product of making the exhaust flow with less restriction.
The cheapest option is to rejet the carb. Main jets often are $2 here, pilot jets are often $5 and needles can sometimes be adjusted to a suitable performance level by moving the C clip to a different position which will raise the needle inside the carb. Your prices may vary given tariffs, shipping fees, etc.
There are some forums that discuss chinese bikes specifically. I do not know the rules on posting those here but I will take a gamble. Maybe someone there has worked on your bike already and can give you a starting point for jets so you do not have to buy a lot of them to test out (you will still have to test some to get optimal performance since its the totality of the system and no two bikes are 100% identical and altitude and fuel quality can also alter performance).
and I bet there are others.