About 1,000 miles since carbs were rebuilt by Bing, synced by my local/reputable airhead mechanic, idle set at about 800 rpm. When the engine gets real warm and I quickly let off the throttle, the rpm drop way down, below 500 rpm and I have to blip the throttle to keep it from stalling, the idle will then come back to 750-800 rpm and remains stable. Engine runs flawlessly otherwise. Any ideas why I get the drop and then return to normal idle with the warm engine?