I have a toilet, probably from 1977 when the house was built. When I flush, the tank empties, but the water in the bowl just swirls around. It drains, but the water level doesn't go down as far as it should. Is there an adjustment to the flapper or fill valve, or do I need to replace parts?
Sounds to me like a build of calcium deposits in the j-bend, and or siphon jet hole. Scrape out what you can see being carefull not to scratch the porcelien. Then soak it in CLR, overnight if you can. That will usually restore normal functioning.
If you know its not clogged, then I would check that syphon hole, if thats what that blowhole in bottom of bowl is called. I've pulled my hair out a couple times trying to get a toilet to flush right only to fine some matter in that hole.