One cylinder cuts out at all RPM ranges. When both are working, engine won't idle at low RPMs. Carbs have been rebuilt and seem to operate fine. Points look good, with a gap of .020".
I suspect you have a bad coil. Try switching coils around and see if the problem follows the coil. New coils are about $20/each. Might as well do both and get a tune-up kit (points and condensers) while your at it. Also can't hurt to do those plug wires. After 40 years, you are due for some ignition work.
