The hard plastic gear on the bottom of the flywheel complete broke off,the starter gear looks fine.I bought a new flywheel gear and installed it.After I got every thing put back together and started the lawnmower up,I noticed when you run at full throttle it is not running at full speed.When you engage the blade it gets even worse,it does not run fast enough to cut grass.What could I have done wrong when installing this gear.Thanks PS Troybuilt super bronco 21hp OHV I/C

The part you replaced is commonly called the starter bendix, its designed to be the weak link so it breaks before other more expensive parts can break. Replkacement of the bendix should not affect engine speed. I'd start by check the throttle cable to see if it slipped. When you pull on the throttle does the cam on the carb make a 90 degree rotation, full throttle?