RE: I Found a New Rossi 53 in an old hobby shop!!
Both the Rossi and Webra55(black head) like less than 10% unless you add a head shim or you could be pop'in plugs or worse. For break-in, let them scream (on the bench) with a light load prop like a 9X7 APC (Rossi) 9X8 APC (Webra) bit rich on fuel that has some castor as part of it's oil for three tanks. Up the prop load and fly. The 55 likes a 9X10 max on a fast plane like a Viper.