ORIGINAL: ameyam
With a 13x4, the engine did not have compression after running a few minutes. It did not have the airspeed for any pattern flying either. When I landed I found that the engine could turn over without any perceptible compression. After it had cooled a bit compression was back. This happened repeatedly even with the 12x6. I presume engine is overheating. I have the engine tuned ok, half turn rich from max rpm. I didnt however tach or check the temperature
Ameyam
As I've pointed out repeatedly.... means the engine is lacking sufficient cooling airflow...
Remove the cowl, then try the same thing again with the wider prop... even try the 13x6 prop too.
As said, this engine is more than enough for that plane.... so something is very wrong with the setup.