Do a compression test. When you have a miss at idle, you are looking at an engine that is loafing, and should be running smooth. Unless it lacks for fuel or spark, and I guess, air, unless it has other damage it should idle fine. The valve seats on some vintages of Vulcans get damaged and lower the compression. Unless you have a dysfunctional injector, I think you are looking at a mechanical problem.
Why didn't you start your own thread as the problem is on a different engine and somewhat different?
tom