Hey there guys, I just bought this 86 c20. The 454 hasn't been tampered with still has all the smog stuff dual air pumps and all that. The issue I am having. The choke seems to work. when I press the gas with the truck off the choke plate snaps closed. When the truck warms up and I give it a big rip it idles down. Then I go to drive it. When I get to my destination and put it in park it idles back up really high. I don't know how high because the truck does not have a tach, but it seems really high. Then when I turn the truck off it backfires like a gunshot. I'm a young guy and new to carbs. I have messed with 6.2 diesels but this is my first carbureted square. Any help or advice would be super awesome!