 There is a brass float that closes a valve when the bowl is full of fuel. Two possibilities, both common, are:
1. The needle is sticky and the float doesn"t close it, and 2. The float has a leak allowing fuel in, so it doesn"t rise enough or exert enough force on the needle.
See part numbers 40 and 42 in the diagramme. Though they refer to it as a needle, in these carbs it is more of a ball bearing. Now you will see part number 43 there; this washer is used to set the height of the fuel in the bowl so its thickness matters. The thickness of gasket 42 also plays a part. Floats are either hard to come by or expensive; I forget which. Some have had success repairing them by soldering but you would have to be careful not to end up with it too heavy. I would start by disassembling and seeing if you can"t clean the needle (soak it in carb cleaner; it may have varnish or be gummy from sitting in old fuel). For the float, you can immerse it in a pot of boiling water and if it has a leak the air inside will expand and you will see bubbles come out wherever the hole is. Hope this helps,
Jim
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