Weird issue with gas gauge only on a set of rally gauges. All gauges work when the cluster is not grounded by installing the mounting screws. Install a mounting screw everything continues to work except the gas gauge goes dead and drops down past the E. Any suggestions? 72*birdman

I am assuming that the gauge is hooked up to the sender now. I would suspect the tank ground or the sender is not connected at all. If you have an ohm meter, see if there is any resistance to ground on the green wire from the tank. Should see 8 ohms from a full tank or 70 for an empty tank and in between for anything else.
All but the ammeter gets 5 volts from the regulator can. If the other water temp and oil pressure gauges are working and the black wire to the gas gauge is connected, the only thing it can be is the gauge itself or the tank wiring, the sender or the tank ground.
To test the gauge itself is to ground the green wire with the ignition on and see if the gauge heads for full. If it does, unground it because it is not designed to pass that much current for long.
I think grounding the frame is just coincidental with a sender issue.
Craig

Yes it is wired to the gas tank sender. All the gauges work (Amp, temp, oil pressure, tack and Gas gauge) and read the correct amount as long as the gauge pod is not mounted (one of the 5 screws installed between the gauge pod and the dash frame). Once any of the mounting screws are installed the gas gauge only goes dead all remaining gauges continue to work as before. Grounding the gas gauge with the ignition on does make it head to full.

Do you have the ground installed between the sender and the fuel line?

yes the ground strap is on

the gas gauge itself is grounded to the gauge frame when its installed right? Could that be the problem? I just can't see how it could not be. I have an other gas gauge but am trying my best to not have to pull everything apart again to install it. I tried tightening the cap nuts on the gauge and they weren't bad and the metal piece that goes over the studs was also still in place, so I guess I'll dig out the OHM meter and see what that tells me.

Just for giggles run a ground wire from the sender to a good body ground.

I've seen these fuel line grounds not work so hot

No. The gauge gets its ground from the fuel sender. There is a 5 volt regulator with the three wire spider that feeds voltage to the water temperature, oil pressure and gas gauges. That is working or the other gauges would be broken too. There is a green wire that goes from the gas gauge through the dash harness to a connector in the kick panel. That connector has the dome light, tail lights, backup lights, brake lights and the gas gauge sender wire.
Just short that green wire to ground and see if the gauge goes towards full. If it does, keep going back to the sender in the tank. Short it in the trunk and check again.
Craig

Got the gas gauge working finally. It Did have a bad ground but that wasn't all. I had installed the space saver spare before installing the gas tank and had stuck the sending unit wire and plug through the hole with a grommet on it. when I pulled more wire to plug in the sending unit the spare had the wire pinched and i pulled the wire right where I cut the grommet and it cut the insulation and shorted the wire to the trunk pan. So no matter what I did the gauge wouldn't work, new sending unit no difference. Not till I started back tracking and took everything out of the trunk and looked under the trunk mat did I see the bare wire. That old wire to the sending unit is very brittle and it just cracked when i pulled on it.Laughs on me guys but maybe this will help someone in the future.