If the toilet lever is worn out or not installed properly, it can cause a number of problems. The most common problem is when you have to "jiggle the handle" to keep the toilet from running. The lever may be getting hung up on the water supply tube, thus not allowing the flapper to seat correctly. If there is too much slack in the chain between the lever and the flapper, it will cause a weak flush because the flapper cannot open far enough. This can also be cause by a bad or poorly installed lever, because the lever arm will hit the top of the tank cover. This video shows the proper installation and operation of the toilet lever. Often you don't need a new lever, you just need to make adjustment to the existing lever.