I'm experiencing a similar problem in the '84 115,000 miles. Also, rough idle unless I add a bit of throttle. Use name brand fuel. Have done the purge thing, albeit 20,000 miles ago. Will check for leaks. I understand the RD bolt needs attention if you're idle flucuates, vs. shakes. I replaced it on the SD a bit ago. All filters tended to, this is my baby. Will keep an eye on this thread.

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83 SD

84 CD