If the engine desn't shut down means that there is an air leak somewhere...most likely in the carb...make sure the carb o-ring is ok and fully seat the carb! you should push down hardly the carb against the throat to get it sealed. It is a common problem with carbs that are locked by the method your engine is, as Tower engines, GMS, and many others...don't know exacltly the term.

If it doesn't get better try looking for a leak in the backplate...

Note that if the engine is not shutting down doesn't have anything to do with breaking in time, if the engine is newer it will have unstable idle instead...

Hope this helps,

Jorge