Thanks Bruce, Insanemoondoggie,Hossfly for your fruitful answers.

The nipple is not leaked, now I found out, basically its the fuel line which connects to the nipple of the muffler, ofcourse from muffler exhaust residue comes out, its the fuel like which needs to be put an O ring. Second the other engine I don't think does have remote needle valve, its attached to the carb and it leaks, and yes as hossfly said, put a fuel line pipe to tighten it up so there should not be any risk of air leak or fuel leaking. Thanks again for your responses, I understood it clearly.

Mody