I have a 2014 John Deere 915B commercial mower with a 60" deck. Just lately the engine has started surging as if it were starving for gas. Took it to a friends small engine shop and first thing that came to mind was a carburetor. So, after that it not only surges, it will shut down when the blades are engaged. Have to kinds baby it along with the choke to engage the blades and keep it going. I took it to my local JD dealer and it was there for three weeks and the only thing they could determine was the vent tube which is incorporated in the fuel tank.... was malfunctioning..... $800.00 !! Yes, it's out of warranty so I will be dealing with this bill out of pocket. I don't know much about working on small engines and the components on these newer models. Just wondered if someone out there may have experienced anything like this and what was finally done to fix it. I have done some testing on my own and feel like the vent tube has nothing to do with the action of the engine..... as far as how it's acting up. Been a great mower. This is my fifth year and only 350 hours on it. Appreciate any advice.