Ignition/starting problem

My '63 Dart will crank over fine but not start until you release the key from the start position. I'm currently running a 360 with a points distributor but also had the same problem when I used to have an electronic set up on it.

I've replaced the ignition switch with a known good one and also temporarily bypassed the ballast resistor with no change to the problem. It fires right up when using a remote start button.

What should I check next?