The John Deere Hydro 175 lawn tractor is a riding lawn mower model from John Deere. The Hydro 175 mower bears strong similarities to the rest of John Deere's riding lawn mower line designed for consumers, and as such, the problems underlying its failure to start are common to a host of riding lawn mower models. Fortunately, this allows you to troubleshoot a John Deere Hydro 175 lawn tractor without the help of a professional and without any special tools.
Check the fuel level of your John Deere Hydro 175 lawn tractor's gas tank. Add gasoline to the tank if there is insufficient fuel.
Open the front hood of the tractor and check to ensure the plug wire is still connected to the spark plug. This black wire should extend into the spark plug near the front of the engine assembly. If it is not connected to the plug, reconnect it.
Replace the spark plug if the John Deere Hydro 175 lawn tractor's engine cranks but still does not start. Disconnect the plug wire from the old spark plug and remove it, install a new plug and then reconnect the plug wire to the new spark plug.
Follow the plug wire to the back of the engine assembly. The plug wire comes to rest beside a circular part called the magnetic coil. The flywheel rests just behind this coil, and the incremental gap between the flywheel and the coil is the air gap. Measure this gap. It must be between 0.010 and 0.015 of an inch.
Use a wrench to loosen the John Deere Hydro 175 lawn tractor's magnetic coil if the air gap is not the correct width. Gently realign the coil in relation to the flywheel to adjust the air gap properly. Applying a little pressure to the inside of the outside of the coil should do the trick. Re-measure the gap to ensure your adjustments were precise. This should fix your engine's problems.