If it's trying to cut out during or after a loop then it is too lean and if you leqve it like that you will ruin it. To properly tune a four stroke you need a tachometer since you cannot hear the engine change rpm

BUT, I just realized something, I was not aware that OS made a 61 sized four stroke. Currently they make a 30, 40, 52, 70, 91 and a 120

Are you sure it's not a two stroke OS61 which they do make

It's easy to tell, there will be a black valve cover on top of the head, it's almost exactly square in shape and is locate right in front of the glow plug. Pictures would really be nice or tell us any marking you see on the engine