The pin is jammed in the fully lowered position. Had it disassembled and tried to free it with all manner of gentle persuasion (really just WD40 and carb cleaner) to no avail. I didn't insist further.

At around 2000 rpm, there's a steady stream of fuel coming out the return hose. So pin-out all the way is open valve, correct?

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1968 Mercedes 280S - The Demon Car