I have a Toro Zero turn lawn mower w/ a 16 HP B&S engine. The engine will only run w/ the choke on.

I have changed the fuel filter and put fresh gas in the tank.

A search of the internet has not produced any trouble shooting ideas.

The engine has about 600 hours on it.

Any help?

Try adding about a 1/2 cup of SeaFoam or any good carb cleaner and then run the engine. If it runs well enough to use the mower go right ahead. Your engine is starving for fuel so its likely a clog in the carb. Dropping the bowl and flushing the bowl and needle may also help.