Here is list to check
Make sure tank is up to mark
Make sure flapper stays up for flush
Check the underside of rim and cleanout the holes wirh coat hanger wire
Check hole at bottom and see if hole is clean
Make sure the 1/4″ tube is in the 1″ overflow pipe that fills up your bowl to make trap seal, if it is low it will not flush right.
Take a 5 gal. bucket of water and dump in bowl and see if it flushs okay, if so then you need to do some checking and cleaning.
Art retired plbg
