460 running rough

[PurpleVenom]

After Run N Gun was over, I spent 2 days cleaning and drying out the car Took it out for a drive this past Sunday and noticed the car just wasn't running right. It stumbled on takeoff, ran a little ragged at 2000 RPM and generally just wasn't happy. I noticed this really on the last day of RNG when the engine didn't pull as hard on the straightaway. Took the car in to a local dyno guy to see if we could pinpoint the problem. Well, that didn't take long. Put the car on a scope analyzer and found all kinds of erratic readings. Pulled the distributor cap, and there it was. Carbon buildup on several terminals inside the cap and the center carbon "ball" was flattened out/missing. We couldn't find any remnants of the carbon ball in the distributor, so we don't know what happened. With this missing, the rotor was out of phase and arcing to the terminals. I ordered a new cap (which was covered under the MSD warranty) and installed, and the issues cleared.

Has anyone else seen this? Any ideas what may have caused it?

[carmine]

Hello Purple Venom
Buy the looks of it the carbon ball in the centre of the cap wore out because it wasnt making contact with the spring in the middle of the rotor.
The engine will run fine because it arks across the small gap between the spring and carbon ball.
Over time the arking wears away at the carbon ball till the gap is to large.
Thats when the symptoms you described appear.

To remedy this you must always have about 1/8 preload on the rotor spring.
You will see this when you place the cap on the rotor.
If the cap doesnt bounce up slightly by the pressure of the rotor spring
touching the carbon ball then that tells you its not making contact.
Remove the cap and grab the rotor spring and bend upward slightly.

When the cap has proper contact with the rotor spring it will not ark because there is no gap thus will not wear out the carbon ball.

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[Silverback51]

From what I have read on this site, it's a common problem with the MSD cap and rotor.

[CCX 33239]

It doesn't appear to be exclusive to MSD. I had a similar problem on a Mallory dual point. The carbon center terminal literally disinigrated leaving carbon dust particles all over the inside of the cap. The resultant arcing caused a backfire thru the carb. Unknown to me, simultaneously, a fuel line had loosened and the result was a nasty fire.

[mdross1]

It has happened to a lot of us I'm sure.For years now whenever the cap comes off any of my cars that is one place I inspect.