Myself and a friend of both own 1986 XR250s, and we both had the same problem. I rebuilt his, but I haven't looked at mine yet, but I think my bike has the same problem. It may be common with this year. The problem with the bike that I rebuilt was that the timing chain had stretched and the chain tensioner could no longer make up for the slack. Because the timing was off the piston was hitting the valves and jamming the kick start. Later the kickstart would not jam, but there would be no compression. I had to replace:
Timing Chain: It was stretched
Chain Tensioner: It couldn't make up for the stretch of the timing chain
Valves: They were bent from hitting the piston. I think I probably replaced the valve seals and springs. No sense in using the old ones. Replace them when you have the motor apart.
Piston: The piston may have been good, but it had a big dent in the top of it where the valve hit it. From experience, it's easier to replace these parts than to rassemble the engine and realize that you have to take it apart again.
