Clogged pipes, poor venting and problems with the flushing mechanism can all prevent a toilet from flushing properly, and it's usually easy to diagnose the problem and do something about it. To understand how poor venting and improper toilet operation can affect the flushing process, though, it helps to understand how a toilet works. An old toilet that has become encrusted with lime scale may have reached the end of its service life.
Reasons for Poor Flushing
A toilet flush depends on fast water flow through the toilet's internal trap. When the water reaches the vertical part of the trap that drops to the waste opening and falls, it creates the suction that empties the bowl. This can't happen if water is flowing too slowly, and that can be the result of a clog, negative air pressure or blocked passages from the tank to the bowl that restrict the water flow.
Plunge the Clog
If water backs up when you flush, it indicates that little or no water is draining, which indicates a clog in the waste pipe or the toilet itself. The first recourse for such clogs -- and often the only one you need -- is a plunger designed specifically for toilets. Insert the plunger into the bowl at an angle so the cup fills with water, and then fit it around the waste opening and pump sharply several times. You can block the other drains in the bathroom with duct tape to prevent the energy you generate in the pipes from dissipating to make plunging more efficient.
Snake the Toilet
If you can't clear a clog with a plunger, you usually can with a toilet auger, or snake. The business end of the auger is on the end of a flexible cable. Unwind the cable into the toilet waste opening, and when you hit the clog, crank the auger handle to rotate the head through it. For clogs that are too deep in the pipes to reach from the toilet, find a clean-out fitting in the pipes and insert the auger through there.
Raise the Water Level
A low-flush toilet doesn't need much water to flush, but it needs enough to initiate the flush sequence, and it can't get it if there isn't enough water in the tank. When the fill valve shuts off, the water level should be an inch below the opening of the overflow tube. If it's lower than that, adjust the float level by turning the float screw counterclockwise to bring the water level up. If you have an old toilet, adjust the water level by screwing the float ball clockwise with respect to the armature. This shortens the armature, putting less pressure on the valve, which then takes longer to close.
Unblock the Vent
The suction that empties the toilet bowl would prevent water from flowing through the pipes if the toilet didn't have a vent to equalize the pressure. This vent is connected to a main stack that rises through the roof, and when the vent gets blocked, the pressure is thrown off. You can distinguish a vent problem by the sound of water gurgling in nearby drains when you flush -- that's the sound of air rushing in to equalize pressure. Clear it by physically removing debris from the vent and spraying water into it to dislodge remaining debris.
Deep Clean or Discard
If your toilet is old, and you have hard water, the inlet holes around the rim could be caked with lime deposits, which prevent the flow of water needed for flushing. Physically clean the water inlet holes by poking a 14-gauge wire into them. If the scale is extensive and you can't clear all of it, the toilet has probably outlived its usefulness and needs to be replaced.