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That's the thing. If I disconnect tank wire from the sender and measure it with an ohm meter it pretty constant. If I rock the car I get a little bit of fluctuation from the float. But it never shows an open circuit or a drastic change in ohm reading.

With it hooked back up, I can simple turn the igniton to on and it will either read full or correct (currently around half). If I turn the key off and then back in sometime it reads correctly or sometimes it reads Full.

I just though it was odd that while it was reading full If I momentary grounded the tank terminal, it would ready empty (as expected). Then go back to normal. The suddenly just back to full. This is without any motion to the car. Tapping on the gauge makes no difference either.

It for sure something internal to the gauge since the tank wire appears to be stable. This gauge wasn't hooked up when I got the car and I am pretty sure was out in the elements for awhile.

It seems (per the diagram) that the gauge reading defaults to "Full" if there is an open between the dash unit and the tank unit. (Or anywhere else in the circuit)

If you're certain power is reaching the gauge from the ignition switch, I'd look for a poor connection or an intermittent tank to chassis ground. Another possibility is that the tank unit itself is not grounded to the tank. Adding a ground wire from the tank unit to the chassis might fix the problem. If the resistance element in the tank unit is broken or corroded, or if the moving contact brush is worn or corroded, it could make the gauge erratic. The gauge operates at very low current and voltage, and any poor connection anywhere in the loop will cause erratic action or no reading at all.

What I found on my tank sender was the wiper button to coil was stuck due to crud that had vented into the chamber from the fuel.
You can drill out the rivets and GENTLY split the case and try to loosen the wiper button.
Use CAUTION around the resistive sender wire cardboard. Try not to move it.
Carefullly reassemble.
I ended up mucking up the resistive wire and bought a GM sender and drilled out a hole for the suction tube and added some length to get to the bottom area of the tank..
I have an operational gague that in the 1/4 full to 3/4 full bounces like a 1950's dog head in the back window . :)

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Putting a .01 mfd /400 V paper capacitor between the sending unit terminal and a good ground may help with the wobbling gauge. I've seen capacitors on some fuel gauge senders. I'm not sure whether they're to stabilize the gage reading or to filter radio interference, or maybe both.

Also, yesterday, I managed to siphon 5 gals of old fuel from the tank. That all I could siphon since that was all the gas can space I had. I use that old stuff in the lawn mower and snow blower.

There was probably still a couple of gals of old fuel in the tank, but I'll dilute that with fresh fuel with some Stabil.

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Kevin, I may be mistaken on this, but I believe the 120 and 115 convertibles did not originally come with a boot for the top. Some owners have fitted them having them custom made. They do add a finished look to the car when the top is down.

John,

Thanks. I figured it would be a long shot.

Got the new positive cables installed today. Still need to get the ground straps.

So first shot is new 1/0 cable with a terminal boot hooked to the battery.

Second shot are the cables hooked to the cut off switch. In this shot I still needed to secure the cables to the frame. You can see how the switch is pretty protected in the frame. The front edge is protected by the battery.

Last shot is the new 1/0 cable connected to the solenoid. The old cable is on the left for comparison. Probably 4 or 6 gauge.

Also added the wire boot for starter trigger wire.

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Also, why I was under the car I started to de-grime the transmission. Before I started, you couldn't see any paint on the tranny.

Obviously the bottom plug (yellow) is the drain, and is top plug (red) the fill plug?

Is it similar to the rear end and I fill until it reaches the level of the fill plug?

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Yessir.