6 hp BS on an Ariens mower. Can I adjust the RPM?

I have a fixed RPM mower. The rpm's seem lower than they used to be, and it is not sending the cut grass into the rear bag as well as it used to.
I just sharpened the blade, and it made no difference.
Can I increase the RPM?
I'm fairly handy with motors, so if I thought I could, I'd give it a shot.