Look behind the air filter and on top of the carb. It should look something like this. You may have bumped into a bush or limb and bent the tab inward which would decrease the high speed.

Look at the tab where the spring hooks on the right side. Bend the tab to the left to decrease the high speed.

{{gwi:128760}}

The Briggs and Stratton engine on my Toro doesn't have that tab.

Need a picture.

I'm not clear on whether this is an engine RPM problem or something else. Does the engine speed seem slow independent of how fast the mower is moving? If so the cause may be a carburetor linkage problem. It isn't something to fiddle with unless you can set the rpm to the correct speed.

However if the mower seems to be miving too slowly and barely makes it up a hill and the engine rpm's seem ok, then the Personal Pace drive belt may need to be tightened. This fix is an easy one - simply refer to the owners manual.