The best way to test the shut off valve is to pull the vac line that runs directly to it and test it with your Mityvac (a very handy device that no antique MB Do-It-Yourselfer can live without) you can pick one up at Autozone for about 25 bucks.

Anyway, start the engine up, pull the line for the shut off valve, and give it a tug with your mity vac. If the engine shuts right off, you're going to have to start checking your vacuum system for leaks. If the engine stutters out or simply doesn't shut off, then you're going to have to replace ths shut off valve.

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1982 240D.
My name is Michael, and I am an 18 year old binge-drinkin', cigar-smokin', novel-writin', all around unsavory and unnattractive guy. I tell long-winded stories, and I only shave once a week, although I've had to do it every day since I was 16.