The toilet in one of our bathrooms routinely flushes incompletely. It does not clog or back up, but it takes several fill/flush cycles to get all the "material" to go down. It just seems to be weak overall, like the flush volume is too low. I suspect that this is the original toilet that was installed when the house was built (early 1970s).
Short of replacing the toilet, is there anything I can do?
One thing you can try is to shorten the chain to the flapper. Sometimes a long chain leading to the flapper will not allow you to open the drain fully (you are only lifting the flapper just a little bit). A partially lifted flapper will not allow the maximum volume of water to flow into the bowl in the shortest amount of time possible, causing a weak flush.
I just fixed a problem like this. My flapper had a plastic float on it. The float was waterlogged and would not stay open for the full flush. The float is counterbalanceed by a tube of water that slowly drains from a small hole at the bottom of the tube. Once the water in the tube is drained the weight of the flapper causes the flapper to close completing the flush cycle. The waterlogged float was too heavy for the counterbalance tube of water to keep it open. Replaced it and its fine.
Using the K.I.S.S. formula, my first thought would be to see what is in the tank drain. I've found a Scuba Steve equivalent, a sock and a toilet "breath mint".
If it's the latter, use gloves / eye protection. I got the bleach tablet out with a serrated knife but since I did not have gloves on, I got light chemical burns. Within a month I had fingerprints again so my cat burglar days were done. Sadly, I returned to being a t*rd wrangler.
Could be mineral buildup in the rim's water channel. See this answer: Incomplete Flushing Fix
