This may well be true Martin-the pics below should give you a better idea of the internal workings of the 149 Vibramatic. As you can see-below the venturi is a sort of plenum chamber-the outlet of which is sealed by the steel valve disc-held in position by the return spring-which is itself held in a chamber with two outlet ports. As the piston ascends, crankcase depression causes the valve to move forward, slightly compressing the spring and allowing the fuel charge to pass from the plenum chamber past the spring into the crankcase, As the piston descends, the crankcase pressure plus the spring tension cause the valve to reseat. The key difference I suppose is that the steel valve does not flex or distort in the process like a reed does-it simply moves backwards and forwards against the pressure of its retaining spring. That being said, IMA (International Model Aircraft-the manufacturer of Frog engines and kits) must have had quite a challenge coming up with the spring specifications to achieve just the right tension..........

ChrisM
'ffkiwi'