I must be a dummy! I've read your blog and I've tried just about all suggestions, so.. I must be missing something. Here goes, I have adjusted the chain leaving 1/2 extra, I have also changed the flapper to a universal flapper for American Standard toilet, and still, the flapper closes almost immediately after flushing. In other words, I have to hold the lever down to gat a complete flush. Everything else seems to be working fine in the toilet, except this flapper. FYI.This flapper has a hole at the bottom which I presume facilitates the air flow. Help!

Have you tried taking up slack on the chain?
If the Flapper had a hole in it, the tank would repeatedly try to refill.
I believe there is an air pocket in the bottom of the flapper, so it doesn't close until the water has drained. I have always used cheap universal flappers, but have been told by a Plumber, that there is a number in the Tank or under the lid that will help in getting the exact flapper.
I am not a Plumber, A Plumbing Pro will add to this.

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I have adjusted the chain leaving 1/2 extra,

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That's too much! With the flapper seated you want 1/4" of play. Did you cut the ring off the new flapper before you installed it on t6he hooks? Adjust the linkage and check to see that there's no ring to cut off and you'll be just fine, Good luck, Tom

That hole keeps air in the flapper so it acts as a float. Tighten chain so flapper opens all the way. Is this a side handle toilet?