Irvine 72

I purchased 4* 60 with an Irvine 72. The engine appears to be new and high quality. Two short test flights yesterday revealed excessive vibration (1/2” side to side movement at the front of the fuselage). The prop is a 12-7 APC, which seems a little small. And yes I checked the prop balance.

What size prop and RPM is expected on an Irvine 72? What vibration level should be expected? If the vibration is normal the airframe will need reinforcing at the firewall and front of the fuselage.

Thanks.

Bill