Lastly, if you read the title you would have seen the engine in questing is a YS FZ53.
You got me there, mate. I apologize for misssing that.
Ok, now you're giving us some info that might help. I think PETEC might be onto something with his suggestion that the carb body may be cracked. If it's leaking fuel with the housing screwed in tight and with o-rings in place around the needle valve, then that's about the only thing it could be. Unless there's a possibility that the entire needle valve assembly is not the correct one for the engine., or unless the threads in the carb body are stripped.
Does it "feel" OK when you screw the housing in all the way? Does it feel like it's snugging up well in the carb body? Man, I wish I could see and/or get my hands on this one. It's frustrating not being able to handle it!
A little trick I've used in the past might help you. Take a small amount of your fuel and put some of the two-cycle oil like you add to regular gasoline in it to color it. Different brands have a different color, look for some that make the fuel red or blue, something dark. Use this fuel while you're trying to troubleshoot the engine, the darker fuel may be easier to see exactly where it's coming from. It won't hurt anything since all you're doing is adding additional lubricant to your fuel.
Post those pics if you have them.