Different chainsaws use different breathers but the principle is the same ,with the fuel tank on a chainsaw as you say there must be a breather to let air in to stop vapour lock ,the duckbill can't do this as it can't work as a vacuum check valve ,in the image below the tank breather that lets air into the tank would be 18, then you have the duckbill valve 25 to let fuel tank pressure out ,I would think fuel doesn't spray everywhere when pressure is relieved out the duckbill because they also use a filter in the duckbill , the foam filter works by stopping fuel spewing out .
As we know If you don't have a pressure relief on a chainsaw fuel tank ,you would open the fuel cap and get sprayed in the face with fuel after a little pressure built up .
The reason that there are a few people advertising the duckbill for an oil tank breather I think is because the oil tank and fuel tank on some chainsaws are one part with 2 tanks built in so they say it's for fuel & oil tank but just used on the fuel side.
I've never seen chainsaw tank breathers discussed before but just couldn't see how a duckbill could be used for a vac valve but if I've missed something happy to be corrected.
