Hi all! Apologies for the silence, been really trying to wrap my head around this whole lack of thrust issue.
I recently bought a Digital Laser RPM meter to test whether I may have been sold the wrong kV motor (desperate times = desperate measures!). The motor kV the manufacturer specified seems to be ok. The motor maxes out at 5818 RPM @15.1V (380kV). Below is the video of the test.
So the next logical step was to test the motor under load. I attached the 27g 16x5 carbon fiber propeller and run the RPM test again. This time I got only 3337 RPM! The manufacturer chart says this size prop should rotate at at least 4500 RPM, and explains why I’ve been getting only 610 grams thrust per motor at 100% throttle.
I have e-mailed the manufacturer and customer support and they suggest the propeller may be too big, hence the low rotation speed. But wouldn’t too large a prop mean an increase in current draw? At 100% throttle and with a 15x5 prop, current draw is quoted at about 8 amps, yet with a 16x5 prop it can’t even crack 4 amps in my actual tests.
So to dispel the myth that too big a propeller somehow causes lower current draw, I attached 20g of weights onto the propeller (total prop weight 47g).
The motor went from drawing 3.9 amps to 4.8 amps at 100% throttle. So if my 16x5 prop were truly too big and overloading the motor, it should draw over 8 amps based on the manufacturer’s chart.
So, to sum it up, I’m out of options. I don’t believe replacing the propellers will be of any use, and I’m not sure replacing the motors will be of much help either (plus motors are pretty expensive). Kindly advise what I might be able to do. I’m about to walk away from this DIY project altogether 
