How do you know it's the carb that's at fault, what are the symptoms? Often with old bikes rough running, poor pickup etc can be down to bad connections in the wiring loom, or a failing CDI box which can feel kike a fuel problem. For this age of bike you might be just as well to order a new carb from eBay or similar. PM me if you want, I have quite a lot of experience wioth Yam singles (not a qualified spanner tho)