More experiment and testing were done. The hung start occurs at around 30,000 rpm and the problem is associated with the starting gas pressure. I was using large portable picnic bottle with the valve fully open passing thru a small onboard canister and solenoid valve. I got hung start with the aids of external tank with valve fully open while getting normal start without the external tank. If you are experiencing the hung start problem, try lowering the gas pressure.

Thanks.
B777