A Briggs and Stratton engine generally runs at 3600 rpms for the most efficiency and power output. The throttle cable that hooks to the engine bracket may not be adjusted correctly. There should be a small bolt and a plate that holds the cable in place. Push the throttle lever all the way up, and then connect the linkage, making sure that the position on the engine is also running full throttle. Once you think you have it in the right spot, tighten the screw and test to make sure that the throttle cable linkage is holding. Then test run your engine. If it runs noticeably fast, you may have to move the throttle cable back a little. If it runs too slow, push the throttle cable up a little. This is mainly trial and error. If this still doesn't fix the problem, it may be a governor linkage problem. Be sure that all the linkages are hooked up correctly, and be sure that the governor arm or governor linkage is not bent or broken. A tiny spring should also be hooked up.
Let me know if you have any further questions. I would be happy to answer them for you.