A used bike should always be gone through regardless of previous service records or what PO says. A circa 85 needs to be carefully examined, new parts obtained, not to mention New tires and until fluids are changed, etc. The last one I bought, last checked by factory mechanic, had number two cylinder listed at 80 psi compression and other stuff. Valve clearences were way off. After adjusting valves all compression/leak down was great, above what an old bike should be.
The PO asked what I was gonna do with it? I said, "Take it apart." Oh, and do a lot of research/reading, you tubing, manual buying. Oh, spend a lot of money.
Or take it to a competent mechanic. Or buy a new one.