All the G26 engines I have run (8 or 10) will turn a Mejzlik 18-6 around 9000 rpm here at 7000 feet without the muffler..The stock muffler cuts this to 8200...The numbers are lower with other props..I have no 18-8 to test..I have personally held the spring scale on the tailwheel of a plane in Phoenix, AZ, altitude 1000, that read 17.5 lbs pull...I then flew the 14 lb plane it's on, flew very well....Should be very good on a 12.5 lb airplane....All these engines were NIB, very little run time......Try it without the muffler
The same engine turned 8800 using a Dale Abell G23 muffler, 8500 with a Slimline Pitts style......