Does the engine have normal compression both before a run and after? The symptoms you describe could be caused by a stuck ring. Looking in the exhaust port, how clean is the ring land area and skirt on the piston? Another possibility of the cylinder base gasket being bad or the cylinder just not being tight. It might be a good idea to remove the cylinder to inspect the piston, rings, ring gap, etc. and replace the base gasket in the process. Any oil residue along the crankcase parting line? That area could be a source or problems. The engine might just be worn out and need rebuilding.