A local (in Alaska) Suzuki ATV independent expert tells me that the intake valve itself if the culprit, I guess it's head gets weak and starts sinking into the seat. He said I'd better replace it before running it more because it can get ugly if it fails.

 

see explanation and photos :
'87 LT4WD - valve lash disappears - Suzuki ATV Forum

 

by the way, if you need a new head look into used ones on ebay, they could do the job, i would pick up a used head and new valves and see what happens