This is ARC (active radical combustion). It is caused by long (too slow) lean burn that leaves high temps in the cylinder after the piston closes the exhaust port. It is usually leaking crankshaft seals on older two-strokes or partially plugged carb pilot circuits on newer engines.
My KDX200s used to do it when riding in powder-silt in the desert (carb issues) though they were nearly new!
On a 30 yesr old two-stroke you will probably be changing crank seals. The new seals will usually press in with your thumbs, once you get the cases split and the crankshaft out. Be sure to use the special tool that fits over the rod and keeps the flywheels from pinching out of line. You can ruin the crankshaft if you try to get the cases back together without that tool.
I always warmed the cases up to about 150 degrees in the oven before slipping in the crankshaft and sliding the cases back together. It takes two people because the trans has to be held lined up as the cases go back together. You both need good gloves to do it with the oven method.
