What would cause a factory alternator gauge needle to bounce wildly at idle? I started my car and just noticed it is doing this. The motor has a new MP electronic ignition kit, new coil, new voltage regulator, new battery cables and a freshly reconditioned alternator.

Have you charged the battery all the way?

Yes.

That happened to me, turned out the ground to chassis had vibrated loose.
Specifically, the bolts holding the VR. I added an extra braided cable ground strap, no issues since.

good place to start
keep us posted

Easy to get corrosion on the VR bolts. I like the suggestion to run a dedicated ground. Some guys don’t like them because it’s not factory, but I’ve dealt with this exact issue enough to know it’s not 1973 anymore. My current ride had a weird quiver on the gauge in the way home. The ground cured that, but I got tired of watching the mechanical reg open and close so tried a solid state reg look alike for the first time and it’s no longer catching my eye going down the road.

The solid state regulators are the way to go IMO. Many times one can swap the OE cover on so it retains 95% the original appearance

i tried a STANDARD brand electronic regulator when i switched out the original 35amp alternator to a 68 amp unit from a K-car, and it didn't work, although it was wired correctly and it was grounded very well.
as i wanted to get the car running, i switched to a newer style factory electronic regulator i had on the shelf. it works as it should.
as with this particular conversion, slight wiring mods need to be done, but they are extremely easy to do, and look factory when done correctly.
amp gauge doesn't move much when first fired, but settles quickly to zero when running.
i have an auxiliary volt gauge and it shows right at 14.1, although at the battery it shows 14.3. this can either be the gauge or the wire size or both.
not sure if i got a defective unit or not, and i just tossed it on the shelf with a piece of tape saying "no good". the return window is way past, so the only way i can verify it as being bad is to get another one and see if it works as it should.
i may do that some day just to see, but i'm in no particular hurry to do so.

Why would you think he has a mechanical regulator? Not like he bothered to fill in any details.

"Originally Posted by SomeCarGuy
Easy to get corrosion on the VR bolts. I like the suggestion to run a dedicated ground. Some guys don’t like them because it’s not factory, but I’ve dealt with this exact issue enough to know it’s not 1973 anymore. My current ride had a weird quiver on the gauge in the way home. The ground cured that, but I got tired of watching the mechanical reg open and close so tried a solid state reg look alike for the first time and it’s no longer catching my eye going down the road. running a dedicated ground wire."




i forgot about the dedicated ground wire. you are correct sir.
one could do that modification very neatly, tying it into the stock harness carefully, making it virtually invisible.
it would just take some time and thought to do so.