Lotus Europa Community

If only one tank is leaking, there will be more water in that tank, water from the bottom of that tank up to the outlet pipe, and half the amount greater than fills up to that outlet pipe, with the other half flowing through the connecting pipe to the other tank.

Unlikely that both tanks would present as leaking at the same time IMHO.
If I've understood this correctly, yes, but I'm not sure what you mean by "fills up to that outlet pipe".

But interconnected tanks will certainly syphon both ways.
F'rinstance, assume there's, say, one pint of water accumulated in the left tank while the car has been parked over night.
(OK, technically, it'll start to syphon immediately the rain water starts to get in.)

If you now park the car on the right side of the road, you'll find the water has syphoned itself to the right tank. This assumes there's a reasonably steep crest in the road.
Similarly, if you then move the car to the left hand side of the road (facing the same way) you'll find the water has moved back to the left hand tank.

I recall going to a BBQ somewhere and parking on a slope. When I later stepped into the car to go home, I noticed the fuel was low but rose back to normal along the way.

There's also "Water Finding Paste". Our guys used this stuff to keep track of water ingress to an underground fuel storage tank we had, back in a past life.
Here's a link to an Oz product (and demo) but it must be fairly common.

https://www.fuelequipmentspecialists.com.au/water-finding-paste/

Might be a part or three missing as well.

What is the square depression and cut-out in the front plenum area for?

But still, a nice looking survivor - just too bad whoever wrote the ad glosses over so much. Or maybe has no idea?

Ron

I suspect they have no idea, but I think such assumptions are quite unbecoming when flying the Hemmings banner (though, having said that, my respect for Hemmings as a pillar of the collector car community has diminished significantly over the last few years).  The word 'original' is tossed around quite a bit in this ad, but there are many non-original features of this car including the paint, interior, and engine (or at least the cylinder head). Not a big deal, but it needs to be presented as such.

Besides, the ashtray is missing, so it's really just a parts car 

If you haven't done so, how about just draining the tanks dry, isolating them, and see what happens when it rains?  You don't need to drive the car, right?  I would imagine only one is a problem.  Then take it out.  inspect it carefully, then if necessary test it with compressed air (not much, like from the exhaust of a vacuum cleaner) and a bubble solution to find leaks.  Need to plug that little breather nipple on the inlet, of course.

I have been giving this a lot of thought.

If only one tank is leaking, there will be more water in that tank, water from the bottom of that tank up to the outlet pipe, and half the amount greater than fills up to that outlet pipe, with the other half flowing through the connecting pipe to the other tank.

If both tanks are leaking, and one is leaking greater than the other, I will only find this out after I seal the top of the leaking tank, or the tank that leaks more.

The right hand tank had more of the 1 pint in it, (there was very little in the left tank) so I will start by sealing the top of the right hand tank.

I further deduce that to fill an empty tank up to the outlet pipe needs just over 1 pint, which seems about right.