For a while a 25 -- 28 hp ( 19 -- 21 kW ) Ava 4 was trialed but rejected as unreliable .
Ava 4 is an air - cooled flat four engine .
For a while a 25 -- 28 hp ( 19 -- 21 kW ) Ava 4 , an air - cooled flat four engine , was trialed but rejected as unreliable .