Poor mileage on '81 300SD

My '81 300SD is giving very poor mileage, latest filling showed about 19 mpg. A recent long trip produced only 24 mpg. Also, on cold starting and heavy acceleration, the car lays down a smokescreen that would be the
envy of a World War I destroyer. Car was recently serviced -- valves adjusted, filters changed, etc. Runs very well with excellent acceleration. Has been starting in the coldest weather with no trouble. But it should not be smoking like that nor should it deliver such poor mileage. Any thoughts?
1981 300SD Der Klunker (ex-Pimpmobile)
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would too much fuel coming in cause this? Injectors? Blow by across the rings? Just guessing here.
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says...

Too much fuel would be my thinking. But the injectors are fairly new.

Don't think so. Woukld that not imply reduced compression which in turn would mean hard starting or no starting on cold days? She has strated immediately this winter on the coldest days.
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Possibilities:
Clogged engine air filter - oil soaked paper element restricts air to engine causing it to smoke. If your engine doesn't have the molded one piece hose from the valve cover vent to the air cleaner, buy it (about $10) and replace the old pipes etc.
Leaking fuel line, check connections to tank. Given the former, the latter is less likely.
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