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Took my old '86 Concours out for a spin today after letting it go unuded for 3 months. It did something it's never done before. When I rolled off the gas, the revs would drop slowly and then itr would barely idle. I turned up the idle speed which helped keep it from dying, but exacerbated the slow return to idle speed problem. It felt a little rough off idle but otherwise ran fine at cruising speed and made plenty of power.
Thinking vacuum leak, rodent nest in the airbox, or a torn carb diaphragm. It is not a throttle cable issue, I'm certain of that. Any other thoughts?