Originally Posted by Me!

This is quite possibly Mav's problem, but it isn't caused by the fuel cap leaking, what's happening is the filler well drain hole/pipe/hose is blocked.
If you lift the fuel cap and look into the well around the filler hole you'll see a small hole, (~3-4mm dia), toward the left. If it's blocked, rainwater sits in the recess on the top of the tank, when you open the cap to fill up it drops into the tank.
It's not unusual to find a significant amount of water in the fuel, but another problem is that if blocked, the metal pipe that leads from the hole to the underside of the tank can rust through from water sitting in it, if it does the tank is scrap as the pipe is inside the tank and not practical to repair. Most commonly the problem is the rubber hose that leads from the stub on the bottom of the tank, right at the rear by the hinge, is kinked. Carefully arrange the hose so that it's not kinked when you drop the tank.
But it's not unusual to find the pipe full of crud too, blow it out with an airline or foot pump, every so often check it's clear and squirt WD40 or a dab of light oil down it every so often to ensure it doesn't go rusty.

If you're going to do this you'll be dealing with fuel, so bloody well be careful OK.

And don't get fuel all over the floor, even if you don't barbecue the garage petrol is a bit slippy and you'll easily end up your backside, so be more careful.

To drain the carbs, lift the tank and take the airbox off, the drain screws are 5mm headed Allen bolts roughly pointing at each other between the carbs as they are mounted almost base to base, open them a few turns and let them drain. Good practice says that you should allow them to drain into a receptacle and then carefully dispose of the fuel, however if you can get anything in there to drain it into you're doing better than me. When whatever was in there isn't anymore, lightly tighten the drain screws, careful with this the carb bodies are made of a light zinc alloy and are very soft, it'd be only too easy to make a nasty mistake here so go easy, they just need nipping into place.

Draining the tank: Take it off and turn it upside down with the cap open, (preferably removed), shake it about to get as much out as possible, then remove the fuel tap to get the rest out, get a half pint of new, clean fuel and swill that around the tank, to make sure there's no water or other contaminants in there, reassemble it all making sure that any disturbed gaskets or seals are in good condition, preferably replace them.