The flapper in your toilet is the rubber paddle-shaped stopper at the bottom of the tank that opens when you push the flush handle. It's a simple part, but it has to be adjusted correctly, or the toilet won't flush properly. Problems with premature closing are usually related to the chain that connects the flapper to the flush handle. You can easily adjust this chain yourself in a few minutes, without turning off the water.
Symptoms of Premature Flapper Closing
The most common symptom of a prematurely closing flapper is incomplete flushing. To generate the suction necessary to empty the bowl, the toilet needs the entire contents of a full tank of water. It's possible that the tank isn't filling to capacity, and it's easy to check that by noting the water level when the fill valve stops. If the tank is full, though, and the bowl fails to empty when you flush, you can pinpoint the flapper as the culprit. Confirm this by removing the tank cover and watching the flapper's behavior when you flush.
Shorten the Chain
In many cases, the problem is that the chain connecting the flapper is too loose, and when you flush, the chain doesn't pull the flapper high enough. For the tank to drain, the angle of the open flapper should be more than 60 degrees with respect to the bottom of the tank. To adjust the chain, disconnect it from the flush handle; shorten it by two or three links, and reconnect it. You don't need to turn off the water to make this adjustment, but it helps to flush first so the tank is empty.
Adjust the Styrofoam Floater
Low-flow toilet design has been evolving ever since federal law mandated these toilets in 1992. In some designs, the proper operation of the flapper relies on a small piece of Styrofoam that floats on the water to keep the chain from getting caught under the flapper when it closes. If this floater is too close to the flapper, it slackens the section of chain between it and the flush handle, and the flapper may not open all the way. To correct this, simply adjust the position of the Styrofoam floater on the chain. It may take more than one adjustment to get the position right, so be sure to test the flush.
Another innovation in low-flow toilet design is the adjustable flapper; if your toilet has one of these, you'll see a series of numbers arranged around the perimeter of the flapper. If the flapper isn't adjusted properly, you may have to reset it. This involves removing the flapper, so turn off the toilet supply valve and drain the tank first. After unhooking the flapper ears from the overflow tube and disconnecting the chain, follow the manufacturer's instructions for resetting it. This usually involves simply rotating the flapper to a designated default position.