It's not a backfire, I'm wondering if because it has no back pressure on it it's just behaving normal. Motors are set up with a load on them. Have you tried it in the water yet? It may be fine.
Often times it's a cheap fitting that comes on the fuel line or did you make sure it fits to a Evinrude?
I assume you pushed it all the way on until it clicked?
Yes, there is a seal in there.
I'm thinking you plugged the intake screen and possibly burned up part of the impeller and the rubber bits are moving around causing havoc.
Pull the lower end and look at the water pump. Should tell the story.
