Jim:

The "butterfly" is probably the anti-reverse-running valve -- weighted to stay open when the engine runs in the normal rotation, but closes and chokes off the "exhaust" to prevent the engine running backwards. It isn't connected to the throttle system at all. Diesels can run (rather poorly) in reverse rotation, usually caused by stalling under load (as it braking hard in gear). Rather scary to step on the accelerator and go backwards! No oil pressure, either, as the pump is sucking the oil out of the engine instead of the oil pan. MB put a valve in the intake in 1961.

I don't think the later IPs inject while running backward, or some other such means of preventing reverse rotation running was incorperated rather than the butterfly valve.

Older diesels with vacuum governors (up to 1975 300D) also have a standard "butterfly" to produce the vacuum to control engine speed. Later models use a mechanical governor in the IP instead. Maximum engine speed is set via a screw on the butterfly link at the intake, idle speed by the length of the link over the valve cover. Butterfly should be nearly closed at idle, stop screw sets maximim opening.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!