engine idleing too high ( questions )

[sam's 72]

I started up my new 383 yesterday. I have a 750 Holley HP series carb. NEW! MSD ING. When we started it up, it idle at around 2200 RPM's, started trying to get it down. It came down to 950 rpm's, and would not get any lower. I have mechanical advance billet distrubutor, MSD 6a box blaster two coil. Set timming at 34 total advance. Checked for vacum leaks, by spraying wd-40 around intake, and carb. No apperant leaks. only have three vacumm ports, one for brake booster, one for PVC, and one for A/C controls. All hooked up. I am running a air gap intake, with one inch spacer, spacer was drilled and taped for vacuum port for brake booster. When you go to shut it off, it wants to run on, and right before it dies, it blows out hard through top of carb. Every things seems right. The engine fired right up. What do you think this could be? I am leaning towards, carb and timming. The timming light we had was a digital light. Not sure if it is accurate with this MSD ing. Going to put my light on it today and see if the timming is correct. I am out at work today, so it will be late this evening before I can get back to it.
Sam