I bought a rotted out but running and driving 66 wildcat for parts for my project. This was a 401, didn't run great but was able to cruise with it a bit. car idled ok. When I brought the engine in for rebuilding the block was bad. Found a 425 block and did a complete rebuild to stock. Used the heads and manifolds from the 401. I have a pertronics kit in the distributor. Ignition timing is set. Cam is stock replacement, cam timing checked and correct. Carb is a rochester 4GC. I rebuilt the carb myself. I used a well-recommended kit and followed the manual. I am not a professional carb doctor, but I have done a number over the years with good success. Carb has been apart 3 times to make certain everything is correct. Engine starts right up and runs great unless you try to get it to idle down. In neutral, about 700-800 is the best it will do. Any less, it will start surging and die. If you put in gear, it will die. Idle must be set at about 1100 for the engine to stay running in drive. It's like the idle circuit is not working. I figured at first it must be a vacuum leak but have not been able to find any. I even pulled the intake manifold off and pressurized the exhaust section, to see if there was a leak from there into the intake. With the rpm up, the engine runs great and has plenty of power. I have don't know what to try at this point. If anyone has any ideas, I'm listening.