What are the common reasons the 15hp outboard won't start?

 

welcome to the forum spark, air, fuel basic needs for ignition. possible spark plug and or lead not good, no compression, air filter clogged not enough air, no spark coil bad.

 

More info, pls. Recent problem or after sitting over the winter? Pulled and replaced plugs? Runs on starting fluid shot?

 

Runs on starting fluid indicates that it is a fuel supply issue
On some motors there is a valve that gets turned to shut flow of fuel to motor
It is useful especially when putting motor into storage for winter
If not the problem then need to check out delivery system