so this has been driving me insane for about two weeks now. running a 5inch freestyle build, t-motor f40 pro on all four, 35a blhelis escs. three of the motors spin up perfectly smooth but the rear left one stutters for like half a second before it catches up and spins normal. once its in the air its totally fine, no issues, no desync, nothing. its just that initial spool-up that feels wrong.

ive already swapped the motor out for a spare and same problem. re-soldered the phase wires, checked continuity, reflashed the esc firmware, even moved that esc to a different arm to see if it followed and it did ? so it seems like its the esc not the motor. but i replaced the esc too and STILL the same thing. i dont get it.

the only thing i havent done is look at the fc output for that specific motor. could this be a signal wire issue? or maybe the pad on the fc is messed up? kinda stumped at this point.

yeah that fc pad thing is worth checking ? ive had a cold solder joint on a signal wire do almost exactly what youre describing. looks fine visually but under a loupe you can see its not actually bonded properly. reflow it and see what happens. also worth checking your motor order mapping in betaflight, make sure motor 4 or whatever is actually mapped to the right output and not sharing a timer with something weird.

the fact that swapping both the motor AND the esc didnt fix it really does point upstream. could also just be a dodgy wire between the fc and esc, those little 26awg signal wires are fragile as hell especially if the build has been crashed a few times.

did you check the motor direction settings after you reflashed? i did something dumb like this once where i flashed new firmware and the motor timing had reset and one was running different timing than the others. probably not it but took me forever to figure that out lol