You need to adjust a toilet's water pressure when a toilet takes too long to stop running, due to the extra amount of time it takes a tank to refill after each flush. Water pressure is also important in providing a full flush, as a toilet needs the most water entering the bowl at the most speed to help create the siphoning action that many toilets use to rid the bowl of objects. Too low a water level will contribute to what is called a "lazy flush."
Turn the shut-off valve clockwise in order to increase the water pressure entering the toilet, and increase the speed at which the tank refills after each flush. The shut-off valve is usually found directly behind and left of the toilet, approximately 6 inches above the floor. Lower the water pressure by turning the valve counter-clockwise. Turn the valve completely counter-clockwise to fully shut off the water pressure to the toilet. Turn the valve completely clockwise to allow the greatest water pressure.
Remove the lid to the tank. Flush the toilet and determine if the fill valve is allowing the tank to refill sufficiently, even with the waterline mark usually found on the back wall inside the tank. Replace a damaged flush valve that is not allowing enough water into the tank. This lack of water in the tank will not allow enough water pressure for a full flush. Shut off the water to the toilet. Flush the toilet, holding down the handle to allow as much water as possible to drain, and soak up any remaining tank water with a towel. Unscrew the supply line from the bottom of the threaded shank of the fill valve underneath the tank. Remove the fill valve and install the new fill valve. Connect the supply line. Turn on the water to the toilet, flush the toilet and make sure the tank refills to the proper height. Check for any leaks.
Bend the arm of the float ball slightly up to raise the tank water level, if one flush is not strong enough to fully empty the bowl. Raise the water level if your tank has a water-intake assembly by pinching the clip attached to the thin metal rod and sliding the clip and the cup upwards. Slide the clip up about an inch at a time.
Flush the toilet and determine if the flush ball or flapper is closing too fast. The flush valve should stay open long enough for enough water to drain into the bowl. Replace a damaged flush ball or flapper. Make sure the chain is not too long to allow the ball or flapper to open far enough.
Shut off the water to the toilet. Dump a bucket of cold water into the bowl, all at once. Absorb the small amount of water that remains in the bowl, using a towel. Wipe the bowl and rim dry with the towel. Cover the jet hole in the bottom of the bowl, along with all the rim holes in the bowl, with strips of a heavy duct tape. Remove the tank lid and pour white vinegar into the tank's overflow tube. Allow the white vinegar to set for a while in the channels and loosen any mineral deposits which may be clogging the holes and affecting the water pressure of water entering the bowl. Remove the duct tape. Straighten a wire coat hanger and poke the end of the hanger into each hole, moving the hanger back and forth, in order to remove the deposits. Flush the toilet.