My sons honda 110 started backfireing. I checked it out and found the intake valve had been hitting the sparkplug. We have a parts 110 that I got last summer that I thought I would rob the head off of, but after gettting it apart it had the same problem.They both had the correct plug in them, and the parts bike had to be like that when i got it because its never been messed with scince I got it. Anybody have this problem before?