Check to make sure that the throttle plate is closed completely when at idle. Engine should just about stall if it closes completely. If not, cup your hands over the airhorn to block off air to the carb, if the rpms increase you have a major vacuum leak for sure. If you have a set of bake line pliers, clamp off the power booster line to check for rpm change also clamp pcv vale hose to check for a pcv valve sticking open. If you still haven't nailed it down use an aerosol bottle filled with water or Windex or any other non flammable liquid and squirt along the intake manifold where the ports mate to the heads. Along the carb base gasket and any other vacuum sources. Do not use Carb cleaner or any other flammable. Use common sense. Why would you spray something that can catch fire on a running engine? The whole point is to make an rpm change at the leak source. Carb cleaner will increase the rpms and water will lower the rpms same test result.