Hang on to 5000 tours

Hello my TS 50 is hanging at 5000 tours then tortures itself up to 6500 tours and then pulls off like sow.About and below everything is great.Spring great and running also your 70. Have tried everything mixed fat lean without success new spark plugs new crankshaftsimmerings and bearings motor completely resealed and trotzdem she almost stops in the speed range and takes no gas at I get her just by shutting down from the hole, am with my Latin at the end has a an idea