I've been noticing that on some days my idle speed will fluctuate up and down. When I popped the hood, I noticed the shut off lever was moving up and down, and when I held it steady, it idled fine. What would be causing this? I don't think it's anything major, but it is kind of annoying. I'd like to up the idle speed somewhat which I think would fix it, but I can't figure out how that idle adjuster works. You can't rotate it, but you can pull it out and put it at a different position, but I tried that and it didn't seem to do anything to the idle. Anyone know how that knob works?

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87 300D - Running on Veggie oil; 260,000 plus miles; Original #14 head