So after some work and cleaning I've brought my 461707-0147-E1 19.5 opposed piston Briggs back to life. Well after running for 5 min I hear this ratteling sound and notice some smoke. Up on removing the cover from the valves and piston I find that my exhaust valve is loose as in no longer attached to the spring.The keeper (494553) is laying in the bottom of the valve compartment. Any ideas on what could cause this to come appart?