76.ta;
The NC is a nice beast of a motor and is quite easy mechanically compared to the other YS's. First off is the timing correct? With the piston at TDC, the cam lobe has a dot on it and this should be straight up! Secondly as tande mentioned, is your one way valve working properly and is you fuel and pressure lines correct? Sounds like the motor is starving for fuel. The YS line is notorious for throwing props if all is not well. More so than any other motor.
Start with the regulator flush and your hi speed setting was good at 2 turns out.

Something to check is to see if you put the backplate on correctly also. The crank has a key which fits into the fan on the backplate and this may not have been lined up(although yours is running 10-15 sec before she comes apart!). Air leak is also a cause. The ring on the forward portion of the crank was ok?