1) if tightening it didn't help then the connection at the other end of the fitting is the suspect (or a crack in the fitting itself).

2) Is there key-on power at the fitting at the sender? If not, check by grounding with a jumper at the gauge (should go to full) but not for long as that can kill it too. If not, gauge is gebusted (technical term for not working properly). If so, wiring is faulty.
If you got power at the sender but still doesn't work then the sender is gebusted (see above for definition).

3) Sounds like improper float level. Could also be broken motor mount messing with throttle linkage (worse one direction than other?). Check for mount by jacking up engine (board between sump and jack, please). if it lifts off, then mount kaka (another technical term).

4) Hoo, boy. start with linkage dis-connected from carb for bends, kinks or missing bushes. Lube all moving parts. While dis-connected check carb itself for similar issues. Worn throttle shaft bushes can cause a hitch in the git-along of a weber. Make certain all throttle return springs are OK and properly attached wherever they need to go.

#3 and 4 can certainly be related.

Personally, I'd get the SUs rebuilt and go back but that's my 2-cents.


--
Mike!