Your toilet flapper is probably misaligned with the flush valve seat that it sits on. The flapper (or stopper) needs to sit perfectly on the valve seat (also called a valve or flapper gasket) for the toilet to stop running.
The toilet flapper valve adjusts to match original toilet settings and eliminates water waste by decreasing the amount of water flushed. Ideal for all standard 2 inch toilets, including: 1.28, 1.6, 3.5 and 5 gallons per flush (gpf).
The problem is caused by too much slack in the lifting chain that connects the flush lever to the flapper. When the chain has too much slack, it cannot lift the flapper high enough to allow the full amount of water to flow down through the flush valve; it closes prematurely, stopping the flush.
The chain attachment fitting on the valve frame is the center point. Rotate the flapper body from "1" for the quickest flush/lowest water volume to "9" for the longest flush/maximum water volume, with "4" as the standard setting.
Look at the flush valve drain opening at the bottom of your tank. If the opening looks about the size of a baseball or orange, you need a 2″ flapper. If the opening looks about the size of a softball or grapefruit, you need a 3″ flapper.
Moving a float (a float is placed on the flapper chain) up the chain, so you are setting the flapper to close faster. By lowering the float on the chain you are causing the flapper to stay open longer.
The average toilet flapper lasts about 4 to 5 years. It is important that you replace your toilet flapper when needed to prevent damaging leaks.
Too much water in the tank
By far, one of the most common causes of twice-flushing toilets is that the water tank holds too much water. Excess water in the tank creates an overflow, and when flushed, it refills quickly while continuing to discharge water into the bowl.
Wipe up any water left inside the tank with a sponge. Lift the flapper and prop it against the overflow tube. Apply a light coat of Vaseline around the rim of the flush valve opening. Lower the flapper and restore the water supply by opening the supply valve.
Add weight to the flapper: find a half-inch nut, remove the chain from the lever, put the chain up through the nut and re-attach it to the lever. The chain will keep the nut from getting in the way of closure.
Turn the handle clockwise to turn off the water. If the valve will not completely turn off, you can still proceed because it will not effect this simple replacement of the flapper. Step 3: Remove the water in the toilet tank. The water does not have to be completely removed, as long as it's just below the flapper.
If the chain isn't tight enough and the flapper doesn't go all the way up, attach the paperclip to a chain link closer to the flapper valve. If the chain is too loose and the flapper doesn't completely seal the hole, move the paperclip to a chain link closer to the toilet handle or add another 1 to 2 paperclip links.
To check for a leaky flapper, take the lid off your toilet tank and flush the toilet. Watch the toilet flapper while the toilet flushes. A leaky flapper doesn't reseal properly after the toilet has been flushed. If your flapper is worn, broken or damaged, replacing the unit should help your toilet function normally.
Make sure you have enough slack and it is not holding the flapper up. If this is not the case, feel around flush valve (what the flapper sits on) and see if you notice anything that feels not smooth. You may have to replace the flush valve if the flapper cannot make a good seal on it.
While flappers are simple devices, they are not indestructible. Over time, the flapper can warp, crack, or break. If it does, it is no longer able to control water flow between the tank and bowl. Accordingly, water often leaks from the tank into the bowl, creating an annoying, trickling sound.
A majority of toilets will use the two inch flapper; however three inch flappers have become popular in recent years and can be found in newer toilets manufactured since 2005. An easy reference for determining your toilet's flapper size is to look at the flush valve drain opening at the bottom of your tank.
With the old flapper removed, you can now install the new flapper. If the flapper has a ring-style design, then slide it down over the overflow tube, before connecting the new chain to the flush handle lever. If the flapper connects with ears, then slide each ear over the pegs on the sides of the overflow tube.
With a simple adjustment of your toilet flapper, this is quickly resolved. If your toilet is flushing twice, it is most likely due to the fact that it is staying open too long and flushing too much water. If you have an adjustable flapper, this can be corrected by adjusting your toilet flapper to close quicker.