If you're applying enough force to bugger the threads, you're over-doing it!
Be patient. That's the very first ingredient.
Give the penetrating oil time to ... uh ... penetrate.
Heating and cooling cycles make it easier for the oil to penetrate and breaks the bond between the jet and aluminum body.
Go back and forth - tighten, loosen, but don't ever use enough force to bugger the screw head! I know, it seems weird to try to tighten it, and it seems counter intuitive, but it is one of the successful techniques.
I wouldn't suggest tapping on the screw or the body - they're both way too soft!!!
Another thing that's worked for me is aluminum cleaner. I always used to use a Zep product for cleaning aluminum but I can't get it any more. The next best is Eagle1 mag wheel cleaner. Don't get the one safe for clear coated wheels - it's not as strong!
Work on it a little at a time. Say ten or fifteen minutes, and just leave it if the jet hasn't budged yet. I've been able to salvage slide carbs like the Dellortos, with stuck slides and chokes this way. So be patient, it will free up!
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