Try this. I have one Troy Bilt CS4325 mulcher/chipper barely one year old, but just out of warrantee. It sat over the winter and it won't start even after cleaning carb, then changing to brand new carb and plugs.
- Open valve cover and see if one of the valve is stuck. Manually move the valve in/out, bypassing the rockers. If they are stuck, you may have to remove them and clean the rod.
Background: Last fall, it won't start so I cleaned the carb and it started right up. Use for about 1/2 hour to chip some branches. It sat through winter, won't start in spring. Cleaned carb, wont start. Checked everything, even changed to new plug. Check to make sure spark plug fired. Then ordered brand new B&S carb. Still won't start.
Decided to tear apart to see anything apparently wrong, to include removing engine head, just by luck, I notice that one of the valve was stuck (corroded rod, not push rod connected to the rocker) in place. It was the valve rod that you could see when taking the muffler off. Because the muffler attachment opening is horizontal and was on top of the engine, I suspect that water eventually leaked through the gasket between the muffler into the engine, corroding the valve rod in place. The other valve cover, you can see opening when carb is take off. It is vertical opening and is not corroded. My mulcher has been sitting underneath deck all winter. Sometimes covered/uncovered not 100% sure. But, I suspect water from rain through deck and landed on mulcher which eventually leaded through muffler and onto valve rod, corroding it in place.
Hope this helps someone as I searched Internet for a long time and never found this solution. It worked for me. Good luck and stay safe.