Hi!
12000rpm with 15% nitro using a 10x7 prop (what brand) is not particularly good!
An ASP .40 Mk11/Kyosho .40 turns a 10x7 RAM prop at 12700 rpm on 80/20 fuel (Nova Rossi 4 plug and both head shims removed).
Same rpm does a Webra .40 GT using the same fuel and prop.

The old Rossi engines do not like to be lugged down with "big " props! Use smaller 9x6 or 9x7 APC props instead.