I don't know that carburetor type, nomad, but I've attached a line diagram of it, with a crude circle around the needle and seat.

[Linked Image]

The hollow, roughly toroidal object to the left of the needle and seat is the float. I'm told this type of float seldom deteriorates, but inspect it for damage and shake it to see if it has fuel inside it. Your problem is probably just dirt in the fuel intake pipe and getting in between the needle and seat. Before you dismantle the carburetor, turn it upside down, suck on the fuel feed pipe (emptying the fuel out first, or preparing yourself emotionally for a mouthful of petrol), put your tongue on the feed pipe, and see how long it holds vacuum. If it doesn't hold it for at least a few seconds, you need to dismantle the float bowl, remove and inspect the float, remove and inspect the needle, and clean it and the intake pipe/seat area. Be gentle, the seat is delicate. Reassemble and see if it then holds vacuum. If it still doesn't, you need either to lap or replace the seat. If it is a metal seat (some are plastic) try lapping it first, its cheaper and easier. You use something like the lead out of a black pencil as the lap, perhaps putting some extremely fine lapping compound on it, and rotate it in the open end of the seat, where the needle normally goes, with something like an electric drill. Hold it straight, of course, or you'll make things worse than they were in the first place. Don't put any lapping compound on it until you've tried it without, first (graphite is abrasive, and pencil lead is a mixture of graphite and clay). Only lap it with a very light pressure and only for a very few seconds. Inspect the seat area where the needle touches it with a magnifying glass: you want to see an even, extremely narrow seating surface all the way around the seat.

Try the vacuum test again. If it still won't hold vacuum, you need a new needle and seat. We'll need more information on your carburetor to discuss that.