I installed a new flapper and it doesn't stay open long enough. The chain is short to the point not much left to shorten. It was the proper flapper, but just does not stay open long enough unless I hold the handle down. When held down, it flushes just fine. I replaced flapper because old one was leaking and causing run on.
Sounds like the chain is too short. The flapper has to come up past vertical, so that it's out of the way for the full flush. Once the tank starts to fill again, the flapper will slam shut.
If the chain is too long, it won't lift the flapper enough. If the chain is too short, it won't let the flapper fall into the full open position. The chain has to be just right, for everything to work.
I solved this problem by stuffing a piece of pool noodle into the float bulb.
My particular float bulb/flapper valve has two large holes in the bottom and side of a large bulb. This lets air out too fast and it did not stay open. The geometry of the other parts in the tank prevent the flapper from ever settling into a vertical position as described in some of the fixes. No adjustments of the chain length, arm length, or water level had any effect. The flapper always sank too fast. It was like this when we moved in. I noticed that a working toilet in the house had a foam float on it to help the flapper stay open.
I experimented with attaching pieces of pool noodle to the chain, but finally got results when I put pool noodle material into the bulb. I cut off a round slice ("O" shape) that was roughly half an inch thick and cut a break into it ("C" shape but with no gap). I threaded this through the hole in the bottom of the floater. The flapper now stays buoyant for longer and allows the toilet to flush without holding the handle down.
I had the same issue. The flapper would not go up all the way and it was because the holes on the sides were too small and too tight on the little "L" tabs. Macgyvering stuff is the best. Took a drill bit slightly bigger than the holes, driled them a little bigger and Presto. The flap goes up all the way and back down now with no issues.