I have preached before---treat your gauges as a SYSTEM
You have the wiring and "bad connections" part of the system,
the voltage limiter, which can be innaccurate or suffer bad connections
the gauge terminals can have bad connections, or the gauge itself can be out of calibration
and of course the sender.
The sender part "sounds" easy. All you need to do, for full and empty, is to establish at what point you lose suction, or have a little reserve, IE the empty point, and adjust the sender so that an ACCURATE ohmeter reads about 73 ohms at empty.
The fiddle until the sender reads about 10 ohms at full.
The sender is supposed to be about 23 ohms at half.
If you meet either full or empty, the gauge should read thus, and if it doesn't there is then something wrong with the rest of the SYSTEM
Go to rad shack or otherwise obtain some resistors. Substitute a 10 ohm resistor at the sender, or else move your sender to obtain 10-11 ohms, and wait a minute for the gauge to stabilize. If it does not read full, then the sender is NOT your problem. Likewise the empty point If you can somehow adjust the sender to show 73-75 ohms, and the gauge does not produce an empty reading, then the sender is not the problem
Make CERTAIN the sending unit is well grounded to the body of the car.
Bear in mind---you are dealing with 40 year old electrics, which might not have been so accurate when new.
