you may have a vacuum leak. Check for leaks in all joints in between the carburetor and the head.
There are a couple ways to check for leaks, but since you can't keep this thing running, just take off the carb and the intake manifold (if it has one) and perhaps reed valve cage (between the carb and engine, if it has one).
All these parts have gaskets or O-rings between them for sealing the joints.
Your bike's a '78.. almost 30 years old.. and if these gaskets / O-rings are original, one or more of them of them is very likely no longer sealing.
Get a tube of Permatex 2 (non-hardening gasket sealant) and put a thin coat on both sides of the gaskets .. reassemble.. test..