My superglide has started blowing out oil from the breather pipe on the left side of the transmission cover. It's not a lot but its starting to oil up my rear tyre while I'm riding. I've checked the external oil line and its not blocked. Any ideas what may be causing it?

When I rebuilt my motor I had noticed the timed breather assembly bore was worn, the plastic breather drum chews out the cast bore, it's not a major problem and I could have fixed it for $100 when motor was apart, you can hone the breather bore and buy an over sized breather drum! 

You may even have a clogged filter, take ya filter off, drain oil in line to filter( if any) replace filter and see how it goes!