1991 YFM 250b was idleing one minute then quit and wont start. Gas in airbox. Filter is clean. Took carb apart-cleaned-put back on. Still same problem. Motor turns over and plug is firing. Seems to have little compression (finger over plug hole). Has 130 miles on ring job. Could a valve be stuck, need adjustment? What else should I look for? If a valve, is there an easy fix?