Mick,

All tapered-bore engines need a rich, two-cycle break-in procedure.

One must be careful not to take lapped, meehanite (cast iron) piston, steel sleeve engines as tapered-bore engines.

They are not and they need a very rich break-in procedure, with multiple short bursts separated by complete cooling.

The aim is annealing the cast iron piston, to harden it and stabilize its size.