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I have a 2005 250 X with:
Athena 280 kit
2008 ported/polished kibblewhite R head (crf's only)
stage 1 or 2 cam (undecided)
airbox mod
twin air filter / cage
pink wire mod
Pro Crcuit Ti exhaust
A/P pump mod
Adjustable leak jet
stock flywheel and gearing
Racing harescrambles in the southeast (GA, TN, SC, NC, AL)

I just installed the 280 kit and 08 r ported head.
I don't have it running yet.

I had a 13.5 std. bore and 07 r head (not ported)
I had 165 main / 42 pilot / 2.5-3.5 turns out / AP mod/ adjustable leak jet)
Bike ran great - awesome response
Piston and valves were always black - (not nice and chocolate brown)

Please advise on starting jetting recommendations for my new set-up. Thanks in advance.

3.5 turns out on a fuel screw isnt good, really 2.5 out should be the end of the world there. It gets too loose and shakes ,can fall out,can screw up the seat, etc.
A lot depends on what sort of conditions what kind of area you ride in.
My bet is the 165 was way too much main jet and still would be.
I'd install a 158/160. A 42 pilot. 2.0 turns out, ride it and see what I had.
I know with all the mods you'd think more fuel,more fuel.... It's still a 280 though ,and what you had was very rich.
There's a reason JD sells all those needles for 250X's.
If that was my bike, with those mods though, I'd start out about where I suggested and see what happens, letting the plug color help guide any changes. Doug