Replace or change the tension on the spring on the manual control to throttle connection. Most speed problems are caused by the manual cable control cable being maladjusted and one cannot get a mower to run at a decent speed.
Try watching the video in the link below:
were can i find a wiring diagram for a 18 hp briggs and stratton engine
You adjust the tappet. That's the thingy that pushes the valve down.
You didn't say what engine model or carburetor you have, but you can go to http://outdoorpowerinfo.com/repairs/ and see if any of the pictures there look like yours.
Roughly 16.5 HP
You do not state if it is a single or a twin (which it could be of either at that HP) so links to each below:
The Briggs and Straton Quattro is a 4 hp engine
How do I change points to electronic ignition on a 5 hp briggs & stratton small engine?
what is the torque specs and valve lash on a 24 hp Briggs
Timing is off ...
.030
The 14.5 hp ohv Briggs & Stratten engines require a 0.30 in gap