Easy answer…… If it last ran in 1960, the Suez Crisis was in 1956, when our petrol supplies nearly dried up, and everybody was trying to screw the last mile out of every gallon of petrol, fitting smaller and smaller main jets, seizing up constantly, and so on. Lofty Avis even ran his Scott on paraffin !! (Kerosene). If the owner of your Scott found that it was just about OK at mild throttle openings on a 140 jet, he probably just left it in.