Please help me diagnose a power issue on a hexacopter

I am building a hexacopter with APM 2.6 as FC. I got all the way up to my first flight. I went through all my setup checks and everything was fine including a motor test with props off. I put the props on, arm and do my first throttle test. Two of the four motors (M4 and M6) seem to have less power than the others, M4 much less. M4 starts spinning at about 50% throttle, M6 at approx 30-40%, all other motors spin immediately on any throttle input. This was not noticeable in motor tests with the props off.

Anyway I continue to apply power to see it they are getting the same power now they are spinning, but as the point comes where there is enough thrust for takeoff, it is obivous they don’t and the copter is going to tip.

So I exchanged motor 4 with motor 1 to see if it is a motor issue. And this time, the new motor that is now in M4’s place is throttling slowly. So it doesn’t appear to be a motor issue, but something with that arm.

Could it be a poorly soldered connection on one of the power wires? Or an ESC issue? These are the only two things I can think of, and if anyone can suggest ways to diagnose that would be great thanks? The next thing I can think to do is take it apart again and re-solder the two arms and see if it makes a difference.

Thanks in advance.

Is the copter tyed down during this test?

no its not tied down

I had a very similar issue and found a poor solder connection on the PDB to the ESC of the motor with the issue. When I corrected it the motor behaved like all of the others.

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Thanks. I was thinking this was the most likely candidate as the probability of me doing two poor solders would be much higher than getting two faulty escs