I had a minor crash (low speed, tips of a few props clipped off, no other visible damage).  I did not fly it again right after the crash.

I bought new props and some prop guards which I installed.  Now, when I start the motors with the combination stick command, the rear starboard prop spins very slowly.  I tried taking off once and it immediately tipped over.

I tried swapping the prop with the forward port prop in case it was somehow the prop, but the rear starboard prop was still the slow one.
I tried unscrewing the prop guards and reattaching it, with no change.
The motor is secure in it's position and when I spin the props by hand all of the props "feel" the same.

VIDEO of the prop speed difference: https://goo.gl/photos/wfgtARfGHo4UU8eDA


Any suggestions? Are there further diagnostics I can run? Is it likely the motor got damaged in the crash? Is it likely I've somehow messed up the mounting when attaching the prop guards?

check the motor by hand turning it and compare it to the others. Could be debris, dirt, grass wrapped around the motor etc. if the motor is clean, then you will need a new one, worse case scenarios you will need a new esc for it too.

If you hear any strange grinding sounds or feel resistance when you turn the motor by hand, the motor is probably almost burned out.
Like the previous poster said it could be the ECS......if that's the case you will have to send it into DJI for repair.

Yes the rear one runs not well. If your motors are clean, I'd suggest you to contact our NA office for fixing :
Email: support.us@dji.com
Phone: +1 (818) 235 0789  
Mon-Fri 9:00 am - 5:00 pm (PST)

"Is it likely I've somehow messed up the mounting when attaching the prop guards?"

I would say yes, probably to long screews, perhaps?

I have one motor fails to turn 1st time. After a few hand turns & 2/3 takeoff attempts it always works fine. Last year I removed hair from it & now think a little  piece remains. I am going to a compressed air canister & try to blast any debris out.

djiuser_eLooqPCwazp1 Posted at 12-12 05:27
I have one motor fails to turn 1st time. After a few hand turns & 2/3 takeoff attempts it always works fine. Last year I removed hair from it & now think a little  piece remains. I am going to a compressed air canister & try to blast any debris out.

Hi there. We do apologize if you are having issues with your unit. Can you please confirm what specific DJI drone are you referring to? Please try to calibrate the IMU via the DJI App and check whether the issue persists. Please keep us posted. Thank you.

This is an old thread.  You can't watch a video to determine a slow motor.  FPS and all that.  You need a tachometer, preferrably with a laser.  Fact is, if the motor was actually running slow, the drone wouldn't fly.  Good luck