Breaking in an engine on the ground is both good and bad .
Good in tha it does allow you to discover basic starting and needle valve procedures

the bad part is that simply letting an engine run on the ground, never permits the cooling that occurs while flying - the engine is acully working les cruising around.
ideally you get a decent needle valve setting which will NOT allow full lean running EVER- then fly, throttling down for a while then up for a while.
But if you ar doing ground running - full throttle should neve be for more than a few seconds .
once yo u find the setting is too lean and heat starts to build - you should throttle back for a while - richen the mix - then try higher speeds again.
gassers should not be run four strokin rich -but an occasional burble is fine. and a slight burble in level flight is usually ' bout right.