Flapper in bottom of toilet bowl bouncing before staying closed

Look inside the tank under the lid, it sounds like the flapper that lets the water into the bowl is getting hung up on something when you flush. It could be the flushing lever. This would cause the flapper to stay open letting water to just pour into the open sewer line. It should be a simple fix. Just see what is binding the flapper & make an adjustment.

Welcome to our new world of water conservation. If you have a toilet that isn't newer....you have something in your trap weir. Or your flapper is malfunctioning. Or your float valve isn't filling your tank correctly. Or your bowl scouring ports could be clogged. There are sooo many things that could cause this....
.New toilets do this because their design to use less water has reached the point of no-longer working to clear the bowl contents. Plain and simple. It has became the norm to flush once for #1's and flush (how ever many it takes to flush all waste) for #2's.
Back to old styles...You can remove the tank top and inspect the water level in the tank(should be about a 1/2" below overflow tube when full), inspect the flapper(flush the toilet and see if your flapper is staying up long enough to allow enough water to enter the bowl to cause a syphon action that ends with a suction sound and complete emptying of the water from the bowl), flush your toilet and look in the bowl to see if the water spraying from your rim is adequately dispensing water to scour the sides of the bowl, and also look at the jet that shoots water from the bottom of the bowl...aimed toward the outlet portion of the very bottom of the interior bowl. This jet is what starts the syphon action that empties your bowl. If you have something stuck in your trap weir....you probably need to call your Plumber to auger the object out or break it down to a size that will pass into the sewer.
The small ports that spray your sides can be cleared with a 3/16" stiff nylon brush...the jet down below can be cleared with a 1" stiff nylon brush. Flapper malfunction is repaired by replacing the flapper. Incorrect water level in the tank can be fixed by adjusting the valve float. but chances are that if it is all of a sudden malfunctioning...it probably needs to be replaced...good luck and always know how to turn your water off before starting work on your plumbing.
Be gentle with the parts inside your toilet tank. Using a calcium remover when clearing bowl inner ports and jets will help tremendously....but be certain to read the container and follow all handling/use instructions you read. Be careful.....Nick

The power of the flush has nothing to do with the fill valve. It has to do with the flapper. When the flapper is opened, the surge of water into the bowl is what causes the toilet to flush. If the flapper closes too quickly, it will take away from the powerful flush. Try to find the correct flapper for that type of toilet and stay away from the "universal" flappers.

You may have to put a weight in the flapper to keep it closed. First try putting pressure in the flapper and let it sit there for a few mnts and check if water still leaking into the bowl, if it does then there could be a crack in the porcelain leaking water or the whole flush valve will need to be replaced. After putting pressure in the flapper and it stops leaking then you just need to put a weight inside the flapper to keep it closed.

My toilet had same problem:
I thought it was a clog in the drain pipe.
But the problem was clogged holes where water flushes into toilet bowl.

Here's how I fixed problem.
Purchase:
Gallon of CLR calcium-lime-rust remover from Home depot.
Pumice stone from hardware store.

Home supplies:
Cut off bottom of empty clorox bottle to make container for CLR
Old 3" paint brush you don't use for anything else to apply CLR
Small cup to pour CLR into toilet flapper opening

Steps:
Look inside toilet bowl under rim and see holes where water comes out
Use brush and apply CLR to holes where water comes out.
Use pumice stone to scrape ... pumice stone will not hurt toilet surface.
Flush toilet to clear holes so far.
Turn off water to toilet.
Open flapper to drain tank.
Keep flapper open.
Use cup and pour dose of CLR directly into flapper opening.
(You can also pour CLR down overflow tube... but flapper opening puts whole dose directly onto problem)
Pour another cup of CLR into opening.
Notice how CLR starts coming out the holes into bowl.
Pour CLR down flapper hole until it runs easy and fast out of holes.
Close flapper, turn on water, toilet fixed.

Most likely, you do not have a good seal on the flapper valve at the bottom of the tank. To confirm, try this:
Turn off water and flush toilet. Then lift up the flapper valve and put a light coating of vaseline on the mating surface where it contacts the toilet tank when closed.
Turn water back on and see if it continues misbehaving. If that fixes it, it was a bad seal. Usually that is all it takes.

adjust the float up to make water deeper....make sure no obstructions around the flapper....flapper needs to rest tight to bottom of inside the tank..flapper is the rubber peice that lifts when you push handle

These models need to have a flush valve flapper that has a styrofoam float on the lift chaninof the flapper. Go to hardware store or home improvement store and get a 'Korky brand toilet tank flapper with float 'ball". Cut the rubber ring piece away from the flapper and you don't need it (see cut here marks on flapper). Install flapper and adjust the float fairly close to the flapper.
When the water level drops in the tank and reaches the level of the float on the opne flapper, the flapper quickly closes causing the toilet to have a "surge" of siphon action which forces the water in the bowl to be drawn out.

MAKE SURE YOUR FLOAT INSIDE THE TOILET IS'NT SITTING TO LOW TRY HOLDING IT UP WHEN FLUSHING THE TOILET FOR A FEW SECS TO SEE IF THIS COULD BE THE PROBLEM ALSO CHECK THE WATER LEVEL INSIDE THE BOWL MAKE SURE IT IS'NT TO LOW