W115

1976 300D ho-hum fuel mileage

Car is getting 20-23 MPG and can not seem to get much improvement. This was the mileage when I started, and have since adjusted the valves (nice wrenches, Kent!), changed the fuel filters, replaced all fuel lines (they were hard and looked leakish), installed Monarch nozzles set to 1620PSI (great kit!). After all of that, it runs like a dream with not much smoke, but still the same ho-hum mileage.

I tested an injector before I cleaned, re-sealed and installed the Monarchs. It popped at about 1400 and shot a straight jet. After cleaning, sealing, shimming and installing the monarchs, they all pop at 1620 and shoot a nice smokey atomized pattern. I was sure this would do it, but there was virtually no change other than in smoothness.

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220D vacuum line tap

I have a '71 220D with a mystery vacuum tap on the main line between vacuum pump and power brake booster.
It's unconnected, and there's also a disconnected hard line up by the firewall, but it doesn't have the length to connect to the brake-line tap. (These are the only vacuum fittings on the car.) That line is split by some sort of check valve or diaphragm/filter, and it disappears under the driver's side fender, right under the windshield corner. It's green, and it doesn't seem to reappear under the dash.
I thought at first it had something to do with the windshield-washer pump, but that's foot-operated, and I've successfully reconnected that.
Any idea on what the mystery line/tap is? I've got some MercedesSource hard line and fittings to reconnect things, but how and what?

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