Rough idle, tach bouncing, '84

[smfinley]

I have an '84 and it has a rough inconsistant idle. It appears to happen more when the car it warm then when first started. It also seems the tach bounces more then the engine RPM is fluctuating. Any ideas what might be causing the rough idle? I also sometimes here a crackling sound coming from the distributor cap, but I changed the cap and rotor and still get the sound occationally, so I changed them back. I could not see any major signs of arcing in the cap. To me it appears to be an ignition issue, but am not sure how to diagnose for sure. Any help or ideas would be appreciated.

The tach will sometimes bounce from below 1000 to up near 2000 and back without the engine really changing speed. It also feels like the engine RPM drops and then catches right before it would die and keeps running. Hopefully someone has seen a similar issue in the past or has some ideas where to start looking.

[So Cal Mark]

if you're hearing noise from inside the cap, I'd take a close look at the pickup unit for frayed wires or contact between the pickup and reluctor

[smfinley]

What is the reluctor? The pickup wires are a couple years old and still flexible. There is a shiny contact point on the outside of the pickup towards the side of the cap. Should it be shiny?

[narfire]

I think... the pick-up that the two wires are connected to might be sliding forward and contacting the "lobes" on the dist. shaft. The pick-up is on a plate that advances with the vacume on the outside of the dist. I have had two dist. where the bushing on the plate is pooched enough to allow the plate to "tilt" forward and the pick-up contact the lobes,giving a rough running or not at all running engine.
Take the dist out and move the shaft. There should be a gap as the lobe passes the pick-up, if not, adjust to spec. (can't remember off hand) but if it still tilts/moves forward ,look for a used dist. or try and get yours re-bushed? Think someone here knows of where this can get done.
Mine was hitting/warn enough that you could determine an angle on the pick-up . should be straight up and down.