I have just replaced the stock air filter on my car and was wondering if i need to connect up the oil breather to the new one or if i can just leave it just venting to the open. Ie no filter. If i do that will dust and dirt get into the system??

I have a 13B with stock Niki carbi.

 

I'd be more worried about condensation forming inside the oil filler tube.

 

Get a small breather filter from an autoparts store - looks like a mini round K&N. Works better than a line to the air filter, in my experience.

 

you can leave it open, unless your in cold climate