This much I can tell you that 16 x4 W is not enough prop for any 120 YS engine a 17x4 pushes the envelope for RPM's .
Theres several things that could make your engine do what its doing,
The Glow Plug
Check Valve
Air leaks
Fuel Tank plumbing
worn out needle valve assy.
Or just plain out of adjustment.
If you can't get it going by doing the simple things I would send it in to YS if your not that familure with there engines.