I believe I keep both sender type seals on hand. Steve (CTR)

Check the sender unit seal also.

so you mean there's no fuel leaking on the sound deadener but there's fuel driping on the floor (??) hmmm(??)

Vent pipe would be my first guess if you are not seeing any fuel leaking out.

Another scenario not yet mentioned is that the rear shock may have come into contact with tank. Seen this several times.

Yeah, the tank will come out and I'll do all the diagnostics. I've been losing fuel and see it in the dirt driveway. It's not the carb.

The flashlight trick is a lot easier to see with the tank out and upside down Mike.

mike are you sure there is a leak in the tank? you said you smell gas but no signs of leak or wet deadener, you might want to investigate on your carb, because you can also smell gas if there is gas leak anywhere inside the engine bay., if there is a leak in the gas tank im sure gas will work its way out thru tiny holes and you will see drips on the floor.

I put a flashlight in it tonite, and see no perfs, but can't see the bottom side. Vent is vulcanized so It will need replacing and likely cracked where I'm not seeing it yet. Sure smells like a British car.

Gonna pull the tank, maybe dunk it in water and see if I see bubbles, then sandblast and repaint. I had painted this with hammerite a couple of years ago, and maybe hid the perfs or sealed them temporarily.

Me too Me too.. 

I had a similar leak that turned out to be a manufacturing defect - just took 30 years to show...  They join the tank halves with a roller welding machine and the operator had allowed it to stray out of line so the edge of the roller perforated the tank.  The defect obviuosly escaped quality control.. However, once painted it held fuel..for years (decades until the paint eventually chipped away and fuel started weeping out. 

As there were only a couple of pin holes I just smeared some epoxy tank sealer over the weld line and have had no more troubles.

I would remove the tank, clean the area adn then assess the holes to determine what kind of repair (or replacemenet) might be best...

Cheers, Ian

Mike, I've had the bottom of the LH tank perforate and leak slowly.  It was pretty obvious that the tank was leaking since there was gas in the spare tire well.

Sounds like your tank is venting into the boot instead of through the pipe it should be and out the bottom.
Better yet...later ones ran a hardline to a charcoal cannister up front.

Check your vent line

That's the way i found the holes in a Moke tank, shining the light down the filler neck in the dark.

It don't take any time at all to remove it. I do it all the time when changing shocks. I have a tank rusted though at the bottom but it sat for years someplace. Remove the cap shine a light down in there and have a look see. Steve (CTR)

spare tire. I don't carry a spare tank in my car.  The spare was an Falken that was one of a pair I bought used.

I've now got the second tire on the rim, but need to solve this fuel leak issue before the tire goes back into the boot.

This tank is not rusted at the neck or top.

Not sure I'm following your post. Yes I have a large left hand tank as well as a Cooper S right hand tank in the boot. The two are tied together at the bottom with a crossover tube. Also at the top with the vent tube which tees and runs out the bottom of the boot. Once in awhile I will be messing about back there and knock one of the tubes off the vent line. It will stink up the whole car. I have also noticed on the late cars with the large tank there is a foam gasket between tand and body at fill. These get wet hold mositure and rust though the fill neck. Strange thing is all the large tanks I currently have are rusty at the top not around bottom. Steve (CTR)

When you say spare are you talking about spare tank or tire?

Any one ever had one of the larger single tanks leak into the boot?  My spare disintergrated and the car smell like old gas, but no sign of fuel in the trunk.  Sound deadening is not wet or smelly.

Gonna pressurize the tank with low pressure and look for a leak.