pware

I have a lawn mower with a 4hp briggs and stratton that's extremely hard to start and when it does start it runs real hard also while im trying to start it the engine will back fire and snatch the cord out of my hand. It acks like its out of time, is that possible. I removed the head and the piston and valves appear to be in sync as far as the intake, compression, power, and exhaust sequence is concerned and the valves aint cracked. any suggestions. this problem started when I hit a piece of tree root