sorry about the starting issues, as I know how frustrating it can be. On a non electronically controlled engine like yours, it is most likely a fuel issue. If your fuel is clean, and no air in the system, how about a bad fuel pump. I have a newer Yanmar with an ECM, and have a fuel pump on mine, and assume you do as well. Can you crack the injectors, and have someone turn the engine over with the throttle full open, while you monitor to see that you have good fuel flow? This is a bit messy, so have some rags around the injectors to catch the fuel.
Aloha SM 72
Port Townsend, WA