I had an os45fsr that was driving Me crazy, doing the same thing, I also had sealed the carb, back plate, new fuel lines, tank and checked the "o" ring on the high speed needle, it ended up being the "o" ring on the slow speed needle ( which isn't really a needle ) but replacing that "o" ring solved the problem.
Just pointing out an other place to check.
Hope this helps.
Here is the link to Your original post
Jeff