Prop_Wash

Hi All, I was breaking in a new OS FX160 engine on the stand, it started up and after a minute I went to full throttle, but when I started heat cycling with the needle valve the engine would not respond. it would just run at the same RPM. My first thought was an air leak but the tank and the fuel lines seemed ok. I was wondering if it could be leaking at the carburetor, or could it be something else?
Does anyone have any thoughts?

Thanks,
prop_wash