John, I remember years ago I had a 71 Corvette with a 350 4 speed that had a high idle. I worked at a Chevrolet dealership in the parts room when I was just starting out. One of our mechanics told me that there was a carbon build up under the intake manifold. I first thought he was pulling my leg because I was young and I won't say stupid, but he insisted. So, I took the intake off the car one weekend, drilled out the plate under the intake and sure enough there was a big chuck of carbon. I pulled the carbon out, screwed the plate back on and put it back together and the car ran perfect. I'm not sure that's what you have going on but maybe someone else can confirm. I hope you figure it out.