hi getting 12,800 RPMS from my ST 45 -on an 11/5 apc- stock muffler-  no baffelthis 45 is a screemer-and not yet broken inmy 51 and 45 are both the italian engines-hard to beat for the dollarsthe needle valves are very sensative on these engines-so a little harder to dial in -but once you get it you have a TIGER on your handsi do not at all care for the power output of the 61 i had -and know nothing of the 75's performance