If the problem remains not tuneable. Go search the engine forum on this engine. I read where some had machining metal left in needle area where it has caused your problem as stated. They just tore it down an flushed, this is where they found fragments. It solved their problem of untune ablity, set rich go lean, thierfore brown caster. Note if fragments are causing lean your very lucky your using caster,last defense on over heating.
Just a thought Rich