Bought a 2012 CRF250X new and have ridden it for the last year. Just now learning I need to check valves. I have noticed the bike backfiring a lot. Intake valve is .004 spec is .005. Exhaust valve is .005 spec is .011. My question is can, I shim the exhaust value and get a little more riding out of the bike before I do valves or is that to far out of spec. Should I shim the intake valve or is the .001 okay since it is within spec. Thanks.