1. When the throttle is fully down three motors are spinning very slowly and one motor doesn't spin. When raise the throttle a bit the 4th motor that was not running start to spin. Can someone advise what could be the problem? I synchronize all motors as in the guide.
2. Is there a way when motors are armed and the throttle is down that the 4 motors will not start to spin?
Hope you solved your problem (long ago!)
If not, the ESC calibration page of the on-line manual tells us that synchronization problems, including some motors starting to spin later than others, is an expected consequence of ESC mis-calibration.
That was exactly my experience. All-new ESCs (4). "All-in-one" calibration (as per the manual page). Result was 2 motors spinning and 2 motors - nothing.
So I manually calibrated the two 'dead' ESCs. Result - all 4 motors spinning, but one starting later and spinning much slower than the others.
So, I manually re-calibrated all 4 ESCs and - problem gone.
Moral (in this case) - don't 'mix' calibration procedures. And keep re-calibrating (as advised in the manual) if you have synch problems.