Bouncy tachometer needle
 

Guys, any trouble shooting advice? Tach settles nicely at idle but has started bouncing wildly otherwise. Crazy all over the place readings...bounces between 2k and 5k regardless engine speed. Where to start looking?

do you have an electronic Tach or a mechanical tach?

likely a ground problem

If you are running an MSD box, this is also a symptom of one that is going bad.

If you can get a spare tach (assuming it is electrical), hook it up. That will quickly tell you with certainty whether it is the tach itself. Might save you some time.

I experienced a similar problem and it turned out to be an inadequate battery to chassis ground (see my post of 7/14/14). Problem came on quickly and went away quickly after fixing the ground. Good luck.

I had the same problem. The ignition module in the distributor just took a hike on me.

If grounds are good,
 

Pmiller176. P, If the ground are clean and tight, the connectors to the MSD module are also tight. You are looking at a signal issue.
What motor are you running?? Does the motor stay steady at the rest of the rpm range?
Lets start at the basics, Check the cap and rotor for being in good shape. Check the center terminal of the cap for not being burnt off and the rotor no have a pit mark in it. This is a MAJOR problem with MSD caps and rotors. If in doubt replaced these parts. Next Mark the cap for the wires and rotor location. Mark the base of the distributor to the intake manifold. We are going to pull the distributor and look at the gear on the bottom. We want to make sure that the teeth are not shaved. Mark the dampener on the crankshaft to the timing marks. When reinstalling this you will need to maybe move it a little to get the distrubutor to drop back into the motor.
Look down the hole and you will see the camshaft gear. Make sure the gear looks good. Some of the SB fords are eating distributor gears and camshaft gears. Possible wrong material the gear and camshaft gear are make from. The gears should be about 1/16" wide from top to bottom of each gear. You should see a little wear pattern on both. This is normal. If it looks funny, You have a problem and this is your bouncing Tach. Sorry. Top and front of motor coming apart. Gets hope for everything looks good. If you think it look weird, post a complete of pictures here.
lets say you found no problems with motor. We are back to electrical.
I have run MSD for better than 25 years on different cars, jeeps and now boats. When the stuff works right, no issues, when problems pop up all kind of issues. Msd will repair their boxes for a small charge. MY fix was to get a complete extra system and keep in the trunk for trips. I have used and giving away spare parts for other people who broke down on trips and at race tracks. Replace the distributor module and retest, If problem still there, looking at box. The signal from box is not a steady reading causing bounce.
Last note. Some guys have to run a ballast resistor in the power feed to the coil as to not burn it out if an oil filled one. Question, where is your coil mounted and is it vertical or horizontial?? This is very important with oil filled coils. The coils inside need the oil to cool the windings and if they lay horizontial they can burnt out on the dry side of the windings. Also off the motor helps reduce vibration to it and causing oil to foam inside. I know that guys want there cars to look 100% original.
Lets start with the distributor first. Have any questions call me 732-254-3536. I am in and out all this weekend, have answering machine, sorry no cell. Good luck Rick L.

PMILLER176:

Aside from the tachometer having an internal dampening mechanism to keep the needle steady there are many others things that could cause what you are experiencing. The tach is controlled by the primary side of the ignition, that is the triggered 12 volt source. Check all item that affect the primary side. For instance, ignition switch, grounds, including the one to the MSD box (if used), wires and connections to the coil, the field coil inside the distributor is a prime suspec as it warms up from engine heat or current flow if it has a frayed winding or some rust, the resistance can change and cause this. Lastly an internal part of the Tach could be failing. Stewart Warner in particular were known for this malfunction. Good luck

Tac bouncing
 

I had that problem new ignition coil fixed it.
Cheers, Bryan