I had the same question last week, when i was setting my injection timing on a 1981 300D. I removed the fuel line at the hand pump and installed a auxillary fuel source. I hung a one pound coffee can with a hose running out of the bottom for a gravity feed. A mecanic told me this. He said that the primer pump is only to prime not to hold constant fuel flow. It worked. I got one drop per second. I let it drop this way for 15 drops and locked my injection pump down. It fired the first hit. I hope this helps.