This is exactly why I asked the question. I simply could not conceive of how wearing in parts would help the front half (carb, crank, etc) create a better seal. Of course, a piston ring does develop a better seal from break in, but that has no direct link with air entering the carb.

It's most likely that enough castor oil built up around the carb on my ST that some small leak was stopped. But, again, that has nothing to do with break in.

Still, I can go through this forum and find statements that claim it happens. Look, I don't believe everything I read. In fact, the claims struck me as perplexing enough to merit raising the question.

However, one thing is true... an engine will idle and transition better, and a low speed needle can be set more easily, after the engine has been run in. Maybe this is what people are referring to. Maybe they are not able to set the low speed needle lean enough until the engine is broken in. It could be that with a very rich LSN enough fuel continues to enter the engine that it will continue to run for a while. Though, that would still require an inflow of air from somewhere.

David