Hi I have a 2005 LT 400 Susuki Eiger 4wd I`m having an issue with my exhaust valves I bought the machine new and have had no problems it starts and idles fine but back fires thru the exhaust when accelerated to half throttle I removed the valve covers and I can hear the exhaust valves pop back up after the tappet has come off the valve stemthe valve comes almost all the way except that last fraction of an inch I can feel it with my finger when it does this while turning it over slowly with the pull cord this all started after my son borrowed it for a day I feel he over run it too high of rpm for too long and got it too hot am I looking at new exhaust valves ? this thing only has 2500 miles on it I would appreciate any suggestions thanks