My friend has a John Deere 100 riding mower and it won’t start, after doing some research and checking out my own note, I think there are two possible places that might cause this.

I found these helpful videos and they provide some visual information for this

1. The starter solenoid coil failure
2. This lady did a good job on cleaning carburetor, successful story.
3. There are some common mistakes in starting the engine wrongly.