Have you checked your timing? Also make sure that your carb, I am guessing that is what you have as you never said, isn't leaking fuel down into the hot cylinders after shut off. Also is the idle to high for your engine? Mine idles at 1100 and I have no problems at all with run on, but I did have with one car and found out that the idle had crept up and was about 200 RPM to high. I lowered it and no more problems. Hope this helps, and I am sure that some of the real knowledgeable people will give you some good ideas on what to look for.
Ron
