Guess what's wrong with my bike

So I've been riding my dad's shine for around half a year and everything was going smoothly, the mileage was around 30-35 kml instead of 50-55kml, I also happen to notice that there wasn't any dirty engine oil whenever I changed it, the engine was empty, this happend twice, some friends also told me that the bike made too much noise, and yesterday I noticed there was black smoke coming out of the exhaust, against my parents wishes today I am at the mechanic, they took apart the engine and fixing it now. I was quoted 7-8k rupees.