2 things come to mind. The diaphragm in the carb seems to not be set up correctly. Also if you had a recent crash or propellor strike it is possible the shaft is bent. Either way I would ask Antique. He is the best when it comes to Zenoah engines. If there is any friction on the hub other than when the piston comes up on the compression stroke something is wrong. Again ask Antique.

Regards
Glenn Williams