I had the exact same problem with my Beta 90. Ultimately, I installed an electric
fuel pump and also made certain that the fuel return line had a significant loop that rose above the injection pump. Also, we recently had the manual pump that's fixed to the final fuel filter
fail. Right now we're running without the final fuel filter
- certainly not recommended but we have a big Racor
2-micron filter just upstream.
Good luck, fair winds and calm seas.