So a month back I was riding my bike and the bike had been smoking for a few weeks now. All of a sudden while riding I lost power and it felt like I was in too high of a gear, so I down-shifted. I did this and slowed down and let off the throttle to attempt to see what was wrong. The bike died when I was slowing down, and I could not start it back up. I took it home and ordered a rebuild kit because I knew it needed one and that it finally broke. My brother took the bike apart and found low coolant and low oil, both which I checked before going riding for about an hour. The cylinder was taken off and the cylinder coating had come off in several places, so I ordered a new cylinder. Everything was put back together, timing was checked several times and valves were just slightly out of spec but we figured we would check again after break in. We put everything back together and the bike wouldn't start after many kicks. Then we took the subframe off and verified that the bike was getting fuel(it was). Next we took apart the carb anyway and cleaned it. The second time taking it apart it looked like the valves were hitting the piston top because there were black circles on it. There was a lot of carbon on the valves so we cleaned that and are waiting to put it back together soon. The bike is a 2006 yz450f. Any suggestions on what to do or why it could not be starting?