my buddy's yz450f won't start back now and not really sure what happened. He is new to bikes and doesn't really know much about them so i am looking to figure out what happened to his bike. all i know is what he told me so i will describe it the best i can. we were heading out of my neighborhood on the road going to ride some trails in the area. he was following me so i didn't see what happened. i looked back and he had wrecked. he said that he was in 4th gear and the bike felt like it was trying to change or jump gears. then he said it instantly quit and the back tire locked up. when that happened he grabbed the front brake and wrecked in the road. we tried rolling the bike off as well as kicking it. we have yet to get it to start back 2 weeks later. he said it locked up so i checked that kicking it would roll over the piston. i put the bike in gear and kicked several times and the rear wheel spins. ive kicked and kicked and kicked in neutral and all it does is backfire about every 15-20 kicks. it has fuel, oil, coolant, and since its backfiring i'm assuming its getting a spark from the spark plug. today took the air filter off and sprayed a little starting fluid in to see if that might fire it up, but no luck. i don't know a whole lot about the interals and troubleshooting the engine so i am hoping someone might have some advice. also before this all happened i believe it was running a little lean. when giving it throttle the bike would run find, but when slowing down or letting off the throttle, there was a little bit of a sputter or crackle sound coming out the exhaust. from my understandings, that is carb related which i don't know how to do either. if anyone has any suggestions as to what to check or what may be the problem any advice would be greatly appreciated. thank you in advance.