Thank you all by giving such a fantastic and informative replies.
@sherwan_88
my engine oil light worked just for 2 days after overhaul and now it is not lighting at all not even at ignition switch ON, the electrician checked the light and says that the light and its fuse is okay, but the pressure gauge has gone and you have to replace it, he told me that no need to worry about the pressure and level of oil as it should be okay. brother you informed about oil pump so i will check its functionality as well because i did't check it.
@afarooqi
brother i like this info, and i will definitely clean the silencer.
@mazamwarraich
brother i think you are right that my mechanic is not so good about the engine piston ring changing or he is not honest, because he told me that the valve and its seals are quite good , but i think that he was not so good, and he just want me to come back with same problem but i am confused because it is so early to say something about him at this stage, because currently i am following his directions and orders till 1000 km, so i am waiting for 1000 km then i will send laanat on him hahaha loll. he thinks ALL IS WeLL. after 1000 km i will be free to saying something more wrong about him when he become fails.