This toilet is old, 30+ years. I just changed the fill/float mechanism to a new style, and now the flapper doesn't close when it should.

I think the problem is that the tank now fills so quickly, its filling faster than the water can drain out, so there is never a point at which the water level in the tank is very low ( I think most get as low as an inch or less, at which point gravity allows the flapper to close).

My flapper keeps bobbing about... the water keeps draining into the bowl... which means it flushes once and then a second time (a weaker flush) because water keeps emptying into the bowl after the initial flush.

Any thoughts on what I can do?

I think the problem is that the tank now fills so quickly

I think the problem is that the flapper is made from material that is lighter than previously used. I recently had to replace a flapper in one of my toilets and noticed the same thing. When flapper falls it will bounce about three times. I started to try to fix it but then thought, "What do I care, it makes the toilet run about 10 seconds longer but it seals."

If you are concerned about water usage, reduce the water level slightly and let the flapper bounce. If you want to stop the bouncing you could try running a bead of caulking around the edge on the top side to give it some additional weight.

Something I've also done, is drop a couple stainless steel washers (don't do it with regular steel ones) over the flush chain. As I had some handy and they don't rust.

If this were my call I would replace the flapper. {you did cut the ring off didn't you?) And if that didn't work I would reduce the flow by cranking down on the angle stop.
A few questions. Pump or city water? If pump then what's the cut off point? If city is there a PRV installed? What's your house pressure? Back to you, Tom

Thanks for all the helpful answers.

speedball1: I did cut the ring off. The flap closes no problem with no water in the tank. Its city water, no PRV, no idea what the house pressure is.

I replaced the 30 year old fill valve with a Moen repair kit fill-valve, which also comes with a new flapper.

I think the other responses may be correct in that the material used in the new flapper is lighter, so the flapper doesn't drift underneath the water properly and come to a close.
I should add that tank water level never goes below 4 or 5 inches when flushed, that's how quickly the new valve fills the tank. Either the flapper is too light to sink and close, or the tank is filled too quickly to allow the flapper the momentum to drop down and create a seal.

Right now I have to push the flapper shut after every flush. Otherwise, it will continue to bob around without drifting low enough to properly close, while the water continuously drains into bowl, creating constant low level flushes.

I guess my question is: is it normal for the tank water level to never drop lower than 4 inches after a flush? Or should the flapper close on its own no matter what the water level in the tank is.

The flapper should close when the water level drops below the air cup. When the water level gets that low the air in the cir cup can escape the flapper should no longer float.

The problem as it appears to me, is that the force of the water going out of the tank, not the force of the water coming into the tank, is pulling the flapper down to soon. Being pulled down under force causes the flapper to bounce back up only to be pulled back down. I don't know what has changed in the design or the material it is made of. I suspect the material it is made of probably has more spring to it than the older versions.
A bead of silicone around the top edge should make the edge of the flapper more rigid, eliminate the spring back and stop the bouncing. Simple enough thing to try.

hkstroud: This seemed quite plausible, but I did a test. I fished out the old previous flapper, which is heavier and clearly designed to be much more rigid, reinstalled it and the same thing occurs.Also, if I hold the float up after a flush so that the tank can more or less empty then the flapper does close. I still think the problem is that the tank fills too quickly. Im experimenting with turning the shut off valve partly closed so that it doesn't allow so much water into the fill valve.

I should add that tank water level never goes below 4 or 5 inches when flushed, Either the water is coming i8n too fast or the water isn't getting out fast enoufg. Try cutting back the volume at the angle stop and let me know if that helps. Good luck, Tom

I closed the water shut off halfway so that the water is greatly slowed coming into the tank, and the flapper is working fine now. The problem seemed to be that the tank was filling too fast.

I still wonder whether the water is also not draining fast enough, perhaps I need to use some iron out or CLR or some such thing to clean out the passageways?

All in all, at least the toilet works normally again. Thanks to all who took an interest in my toilet!

Congratulations. You might consider putting something between the incoming water supply and the drain (a brick) to break up the flow to the drain. If that works the tank would refill an its normal rate. Might try that my self just to see it it works.