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Sorry, late to the party but agree with your findings. Probably fuel injector. I had similar issues, would back off throttle or at idle and bike would die but run ok under load. I cleaned it and it would get better for a little bit but not long. A new injector was the fix. Fuel pump is almost the exact opposite, will idle on a stand and run ok under light throttle but dies under a strong load. You can rev it on the stand as well and since there isn’t resistance on the drivetrain it doesn’t kill the motor...

I had something like this a couple of months ago and couldn't figure out what the hell was wrong for months. To this day I still don't know for sure, but what I did that finally got it to run was to pull the throttle body, and clean it really well, especially the connectors. My assumption is that I had a dirty contact. Other thing I have had issues with on KTM's is the spark plug cap cracking. Took me a long time to figure that one out and it will cause some issues as you described.

Youd be insane to buy anything but a yz65. Hundreds hours of literally bullet proof reliability. Ive personally owned those bikes with over 300 hours still on the original clutch cable. Mark my words, if you buy a sub 2000 dollar ktm 65, youre going to regret your decision week 1.

Brake pedal pivot bolt

HITLIS power valve https://productivepowerllc.com/products/hitlisyz250 DRT head dome A or B https://www.berad-products.com/products/drt-cylinder-head-yamaha-yz250x…

I just reset my TPS to .40 with the butterfly closed and about .56 idle screw in and .66 for cold start. Seems like the bike starts a lot better than before and idles a bit smoother.

Noted, thank you guys. I’ll stray away from straight leaded and do a 50/50, it’s just hard to trust pump fuel in CA these days.