Curvy with movable leak

Hi All,
I've had a search on the forum for leaks, and got a couple of good clues, but found nothing exactly like this.
I have a 2000 curvy (16,000miles) and noticed when I bought it that it had a bit of a stain on the left side of the frame, under the rear of the tank.
Parking it on the side stand led to a very small wet patch on the floor and a smell of petrol, so the obvious thing to do is to take the tank off and look for a leak.
I decided to do two jobs at once, so put the bike up on stands to change the rear shock and realised that there was oil on the bottom of the engine. It had never shown signs of an oil leak before and the oil level was at full, which was where I remembered leaving it.
My first thought was that I had overfilled it a bit and it must have come out of the breather.
However, after it had stood four days (work and all that) it had leaked considerably and the oil level was significantly lower... but no sign of a fuel leak.
Rocking the bike gently on the stands the oil in it looks too runny to me.
Taking the tank off, everything was totally dry with no sign of a fuel leak anywhere.
The next step is obviously to clean everything off and see where it's coming from. In the meantime my first theory (which is probably complete drivel) was that the fuel tap isn't closing fully, one or more carbs is overfilling and leaking into the bore, past the rings and diluting the oil, which is then able to defeat a not too perfect seal around the clutch cover.
Alas, that wouldn't explain the stain on the frame (which did come back after cleaning) suggesting a fuel leak at the tank, rather than down the fuel line.
The tank is now sat on a patch of clean concrete and doesn't seem to be leaking, but that could be because it's in the wrong position or short of fuel.
Does any of this this make any sense to anyone? does anyone have the real answer? Am I just confusing two unrelated problems?
Any words of wisdom would be greatly appreciated.

[Craig380]

Re. the tank, if it's got a pinhole it will most likely be at the lowest corners, so try and stand it on a piece of cardboard tilted up (as if it was propped up with the tank prop on the bike) and see if you get any sign of a leak.

When you look at the oil level in the inspection window, it always looks really thin and runny to me - and that's with fresh oil that's only done a couple of hundred miles. Also, just leaning the bike a couple of degrees from the vertical balance point makes a BIG difference to the oil level you see in the window, I've found.

How much did it leak? If it was enough oil to cause the level in the window to drop noticeably, you'd have a BIG oil puddle on the floor, even a teaspoon of spilt oil can easily make a 6-inch circle if it's dripping onto approximately the same patch of floor.

[johnnyrod]

My tank rusted through at the bottom left side. There was a weep of fuel and when I poked around at the tank it just gave way and dumped onto the floor. Plugged it with epoxy putty and looked for a new tank. Still not fitted it though (hello Ben!).

Thanks for the response Craig, Fair points both.
I had a similar thought about the tank last night, so left it propped up with an oil can. There appeared to be about a half inch spot of damp under the offending corner as I left for work this morning, so it looks like it could be a pinhole or leaking seam there.
As for the oil leak, it doesn't look like enough to have dropped the oil level that much. I put a bit of plastic under it after I noticed the first puddle and I'd say it's dropped about dessert spoon full. It is sat on axle stands under the (inverted) footpegs at the moment, so it should be fairly level, but I know the floor's not perfect there so it could easily be off a bit.
I guess that means that I'm probably looking at two unrelated faults. The oil leak should be fairly simple, but I'm not sure how to approach the tank if I can't find the hole.

Clean the tank thoroughly (near the leak) throw some talc powder on it and see where its weeping from

Good idea on the leak detection. You've reminded me of a trick I used somewhere once before (I can't remember where) which was to use the dry powder type deodorant. It's the same idea, but really easy to spray on an even coat.
Cheers

Maybe try the POR-15 tank sealer kit.
I used it about a year ago and have had no problems since.

Thanks for the advice all. It turned out I'm going blind in my old age, maybe the old headmaster was right!
When I was cleaning the engine to plaster it with talcum, the airline that I was using to blow the water off also blew off some clear silicone that a previous owner had run around the outside of the clutch cover. He hadn't even taken it off and done a proper bodge.
I blasted it with talc anyway (since my girlfriend had gone to the bother of getting it for me) and that seems to be the only oil leak.
The fuel leak is proving more elusive, I think it's in the seam at the back, so might be a bit of a pig to fix. POR-15 sounds like an option, though I have heard tales of it coming off and making matters worse, presumably if not well applied.
Cheers