old westwood t1600 running problem?

I have an old (25 years +) t1600 briggs & stratton twin. Starts fine but after a minute starts to splutter and l.h. plug oils up + if filter off spray is pressured back from float chamber. Assume carb ok or both plugs would be affected. No sump pressure problem. Wondered if faulty inlet valve was allowing pressure to back feed, but with heads off no visible problem.

Any ideas (inc. how I could get at side valve to check further/ where engine number might be found etc/ where an old manual could be got