Belongs to a friend of mine. I'm looking for answers for him. This bang is very loud. After bang, he hits starter again and the bike starts with no problem. Timing issue?
The petcock is probably leaking. Fuel can load up in the cylinders and then when you ignite it it will explode. It's more likely to do it when it's warm because the fuel evaporates and the vapors get into the exhaust and intake. As soon as the accumulated fuel is burned the bike will start normally.
Rebuild the petcock and clean the carb (needle and seat) and you should be OK.
