Engine reving to full throttle on start

I have a 70 Duster with a 340 in it. Was running great without any issues, and when driving home the other day, the engine RPMs just started to sky rocket like at full throttle. I turned off the ignition and coasted to the side of the road to check it out. I check for stuck linkage, and looked for any missing/disconnected vacuum lines, but found nothing out of place. Needless to say, I had to push it onto a trailer to get it home.

When I start it, it immediately tries to sky rocket the RPMs, so I can't really spray carb cleaner around the base of the carb to see if the leak is there.

Any ideas on what else to look at? It is a 340 running MSD ignition and a Holley Slayer series carb.

Thanks