I had a feeling it was a used engine from your post.there can be several thinks to check for,leakage around the carb and intake manifold and leakage at the manifold and head,could be a missing or dried out o ring,check valve adjustment both intake and exhaust sorry I sold my os fs91 and have a ys 91 also valve timing may be off a tooth.if I remember correctly at tdc the indent on the cam gear should point at the valve guides not straight up as most think.post what you find out.