If your MTD zero turn lawn mower will not mulch the grass it may be because the grass is too high or is wet. It is not recommended to mulch grass when it is wet. If the grass is too high, then cut the grass first at a high cutting height and then cut it again at the desired height.
The mower may also have a dull blade that either needs to be sharpened or replaced. Also, the engine speed may be too low to properly mulch. Try putting the mower in a higher speed.
The model and serial numbers on your MTD zero-turn mower can be found on the frame of the mower or under the seat.
NOTE: if you need a part for the small engine in your MTD zero-turn mower, there will be a separate model/serial number for the engine. Here are the small engines that are available in your MTD mower. Visit the page of the engine you have to find out where the model/serial number is located for your small engine model: