I have a Johnson & Starley warm air heating system. The model is Hi Spec?

The model is a high spec J32 and just recently does not want to fire up any more. The pilot light is remaining lit . The room stat is set to max. If I turn off the pilot and the mains power switch and allow to cool for ten minutes and then relight the pilot light and reset , it then fires up the burner and works for a couple of mins or so and then just pilot light again. I am trying to get info on what part I may need to change, regards David.