Could be it just needs a little tap. Of the pressure regulator on the front of the pump, next to the fuel inlet. The internal pressure might be low after all those years. You can check the output at the return line to the tank. I think it is something like one liter in one minute at 1,000 RPM. If you don't get that volume then tap the regulator down ever so slightly and measure again.
Setting the internal pressure correctly has a lot to do with the advance of the timing piston and response to higher RPM later.
Someone verify my numbers as it has been ages since I used them.