I have a twelve year old Wheelhorse mower/mulcher with 16 HP B&S engine. A few years ago the carb became gummed up and it was leaking gasoline. My engine guy rebuilt it and added a fuel shutoff to prevent leaking gasoline in the future. It has been working fine since but now does not want to start. It cranks and cranks and has started the first few times then shuts off after about 1-2 minutes and after that only cranks but will not start. Any ideas why it won't start and what I can do?