If your engine sags at high RPM this is a sign of varnish ( burnt castor oil) build up on piston & maybe sleeve. I find no problem with cleaning off . Use fine steel wool or Scotch-brite. This used to be a real problem with TD .049's & all castor fuel in my "olden days".
Downunder: I don't know what your Avitar is , but is a dead ringer for my mother in law.>>>>>>>>big max 1935>>>>>>>>