Twitching fuel gauges

[ArthurWood]

First the port gauge started to twitch, ie oscillate rapidly, now it's the stbd one which does it intermittently. Can anyone suggest what is causing this and where I should begin looking to fix it?
Engines Merc 7.4l V8 petrol.

 

[Col]

Hi Arthur, how you doing?
Is it just the fuel gauges ? or is anything else affected?
Does it make any difference with engines switched off, or revved up?
Does it make any difference with tanks full or empty?

http://www.arweb.co.uk/argallery/album02/

 

[ArthurWood]

Hi Col. I'm doing fine thanks, tho it's a bit hot and humid here.
It's just the fuel gauges, but they don't do it continuously and don't do it both at the same time.
No problem with engines off. Gauges go to 0. Haven't looked w/ignition on and engines off, but I don't think they do it then.
Noticed at 3200-3500rpm/20-23kt
Seen w/tanks 1/8 thru 3/4 full.
Just had bottom etc painted and boat running like new with >1100hr on ea engine/Regds, Arthur.

 

[Col]

Arthur, it's possible that output from alternators fluctuating, causing the needles on fuel gauges to flicker ( they aren't voltage stabalised like they are in a car )
In your car, when you switch on ignition, fuel gauge rises slowly, with a boat it is almost instant.
You can check by starting and running engines with fanbelt off (Briefly as water circulation pump won't work) and seeing if same result.
Try also with engine off / ignition on to check for fluttering.
Good luck.

http://www.arweb.co.uk/argallery/album02/