I just bought a used snapper rear engine rider. serial number 05010101 . I have put sea foam in the gas but it still does not run smooth. I also realized that the gas tank dumps gas into carb when not running. I have just lost 1.5 gals of gas in the last hour. I know I need a gas cut off on there. What do I need to do with the engine to get it running smooth. It is a 8hp briggs 30" cut. Its just about to make me give up on the purchase and keep killing myself with the push mower. All help and suggestion are welcome. Thanks in Advanced.