How To Check or Adjust the Fill Valve
How to Check the Fill Valve (aka Tank Valve)
Inside the tank is a water valve that controls the water inlet for the refill cycle. When the toilet is flushed, the water is released from the tank and the float sitting on top of the water, drops down, engaging the fill valve to release water into the tank. Depending upon who you ask, you might hear it called a "fill valve", "flush valve", "water valve" or "tank valve". It most certainly is not the flush valve, because that is the part involved with releasing the water at the bottom of the tank. The most correct name is the fill valve, although it is a water valve and it is located in the tank, but from here on, we'll refer to it as the "fill valve".
If the fill valve is misadjusted or defective a common symptom is the periodic or continuous running of water. It may sound like a flush, a mini-flush, running water or brief refills. These symptoms can also result from other problems, primarily leaks.
To check the fill valve, remove the lid from the tank and set it carefully onto a towel. Vitreous china can easily scratch other surfaces, so the towel is highly recommended.
Inside the tank you will see a tall column with some sort of float device connected or attached to it. Typically, at the top of that column is the actual water valve. Running from the valve is a small hose which leads to the overflow tube. The little hose should be fastened so it sprays down into this tube.
The first thing to check is the water level. The water should not be all the way to the top of the overflow tube. If it is, then you need to adjust the tank float to a lower position which will result in less water filling the tank and thus the water line will drop. The valve may have an adjustment feature, such as screw, which when turned raises or lowers the shut-off armature. Try turning the screw to cause the valve to shut off at a lower water level.
If the tank float is set properly and the tank valve is still running, then you will probably have to replace the tank valve. Most tank valves are inexpensive and not terribly rugged, so it is unlikely that the existing valve can be repaired. However, if you are inclined to try, be sure to turn off the water first or you may end up with a geyser. The valve may have a small piece of debris inside, holding it open. A simple cleaning or even a light whack might restore it to operation.