Weak flush: what to check inside the tank first

Check 1: tank water level

Lift the lid. Most tanks have a water line mark molded or printed on the inside wall. With no mark, the usual target is about one inch below the top of the overflow tube. If the water stops well below that, raise the fill valve float: on a Fluidmaster 400A style valve, turn the adjustment screw or slide the float clip up; on an older ball float, bend the arm up slightly or turn its adjustment screw. Never set the level above the overflow tube, or water will run down the overflow nonstop. See fill valves for valve types.

Check 2: chain slack

With the flapper closed, the chain should have about half an inch of slack, roughly one or two links. Too much slack and the handle lifts the flapper only partway, so it drops shut before the tank empties. Too little and the flapper never seals. Re-hook the clip on the trip lever arm to adjust.

Check 3: a flapper that closes too early

Many replacement flappers are lighter or stiffer than the original and drop shut before enough water leaves the tank. Watch a flush: if the tank still holds several inches of water when the flapper slaps down, that is your problem. An adjustable flapper such as the Fluidmaster 502P21 PerforMAX (2-inch) has a dial that sets how long it stays open. On 3-inch valves, use the brand-correct flapper (see flapper size).

Manufacturer reference: Fluidmaster 502P21 product documentation.

Check 4: rim jets and siphon jet

Hard water leaves scale in the small holes under the bowl rim and in the larger siphon jet hole at the bottom of the bowl. Use a small mirror to look under the rim. Clear holes with a bent wire or small Allen key; for heavy scale, shut off and drain the bowl, plug or cover the jets with tape, and let white vinegar sit in the rim overnight before clearing. Background on scale: hard water.