After depressing the flush lever the flapper closes to quickly

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.

Fill the breather hole (1/8" hole) inside the flapper.This is designed to let air out so the flapper will sink. Now it floats longer letting in more water. 25%percent more in my case.

The flapper on these toilets is a water saver. The water saver flappers can be replaced with a standard flapper and I have found this will help considerably. The flapper is located in the tank at the center bottom. It is the part that flaps up and down when you pull the lever.

Toilet runs continuously or doesn't flush adequately

Jiggling the handle might work for a while, but if your toilet is always running or doesn't flush adequately, you need to perform these simple fixes that usually can tame a noisy, inefficient toilet.

Check the Chain

The chain attached to the rubber flapper often becomes loose, causing incomplete flushing and/or continuous running. To fix it, remove links from the chain using needle-nosed pliers until it's tight.

Check Water Level

Improper water levels can also cause problems with your toilet. This can be fixed by adjusting the ball ****, which is located next to the overflow pipe, the open tube to the right of the handle. Adjust the ball **** so the water comes to ½ " below the overflow pipe.

Clean the Surface of the Flush Valve

A continuously running toilet can also be the result of a faulty flush valve. Because the valve is made of rubber, it can decompose or warp allowing water to escape from the tank into the bowl. Clean the sealing surface to see if this will improve the seal. If too decomposed or warped the valve may need to be replaced.

Replace the Flapper

Remove old flapper and replace with a suitable replacement. Attach the flapper to the overflow pipe inside the bowl. Then attach the lever chain from the flapper to the handle lever making sure the chain is taut.

This video will help familairaize you with the flushing mechanism and the repair:

lift off cover and shorten flapper to handle chain/rubber connection flapper not opening all the way

Carefully remove the lid from the tank and set it down on a flat secure surface (lay it on some towels to prevent scratching or nicking. Inside the tank find the chain that runs from the flapper to the lifting bar. Adjust the lenght of the chain so the flapper raises to between 45 and 90 degrees of the drain. You may need to play around with the adjustment to get it just right. You don't need to cut the chain, just move the clip on chain. If this won't work the tank may not be filling to fill mark, which is about 1/4 below the overflow tube. Adjust float to get water to this level.

This is how it is meant to work!
Short push of lever for small flush, push and hold lever for full flush.

the chain should have some slack to it being too tight it dose let the flapper sit properly so itll keep running because water is escaping

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.

See this PDF document link below for a product from Fluidmaster which allows adjustment for the time for the flapper valve to remain open after flushing:

http://www.fluidmaster.com/pdfs/555c_instructions.pdf