Jaffa, remember that the locknut must be loosened with a ring spanner. Trying to use an open end spanner just spoils the nut. This is a good general rule to remember: open enders are special tools for cases where there isn't room for a ring.
It is possible that the lock nuts were over-tightened at the factory, and have not been adjusted since. Never tighten a tappet lock nut to a high torque: it wears and stretches the pitch of the thread, and that makes it nearly impossible to adjust the tappet clearance accurately afterwards. It does not take very much torque to lock them, unless the thread is already out of pitch from the previous tenant's efforts.
You should not be surprised when a chonda runs just as well as a Honda (which means better than any side valve engine): they all do, unless there is something wrong with them.
