yz400f starting problem please help

[shawn10]

hey i bought this 1999 yz400f its not running wen i bought it i checked the cdi and the coil both bad i replaced i have strong spark but its shooting fire thru the airbox and the exhuast is popingplease help

 

[76GMC1500]

Check the valve lash and ignition timing. Make sure the intake valves are closing fully.