I have a 1978 1.5hp with a 12mm Bing carb on it. It will run for a minute or two then it will die out and quit. Has long as I hold down the black button on the side of the carb the one that allows gas to flow into the carb it will run but gas runs right out of the carb. I let go of the button the engine quits. Any body have any ideas or know where I can buy a 12mm bing carb? Thanks Ken