I have yet broken in any BRAND engine that the factory settings where not way rich not only STS! Yes STS richer then most others but better to start out to rich then to lean!!!!! All had to be leaned out at least a little just to keep them running. To properly break in any engine you should wrap the head in aluminum foil and preheat with a heat gun. The STS D21E does not have near the pinch most other STS have but still a good idea to preheat.

Like most engines other then OS take a standard long plug. OS plugs are to short for STS and most other engines. On break in do to running rich good idea to run a hot plug that will all so help with getting temperatures up.