toilet not flushing

One of the 3 toilets in my house doesn't flush completely about 50% of the time. Usually, when the toilet doesn't flush completely, any solids will still be removed but the water just kinds of swirls around without completely flushing. If I reflush (when tank refills) the second time will almost always work OK. Sometimes the first flush will work just fine.
The toilet has been in the house since it was built in the 70s. The toilet is not the newer type that uses less water.
I have tried a plunger, but I don't think the line is blocked. Shower and sink in the same bathroom work OK.
I have had this problem on and off for years but I've just lived with it. Lately, it seems to be getting a little worse.
Thanks for any advice.
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trailer wrote in message ...

"hallerb" in this newsgroup gave this suggestion to me and it works beautifully. Try it, I think you'll be surprised at how well your toilet flushes after doing this. It worked wonders on mine.

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If the bowl is filling up close to the rim then flowing slowly down the drain, then it is a partial clog in the drain and best cleared with a snake. You can have a blockage in the toilet trap which will not effect any other fixture.
If it fills incompletely but otherwise flushes with diminished water supply then the flush valve needs adjusting so that the tank can fill more or the flapper valve needs replacing or the chain to the flapper valve needs to be shorter so it can open all the way.
Another possibility is that there is enough water but it is restricted between the tank and bowl so it moves slowly. This is usually caused by lime scale build up (do you have hard water) (or occasionally debris dropped in the tank) and can be cleared with acid, CLR, vinegar etc added to the tank and flushed through a few times. A very bad case can be cleaned by digging out the water ports along the rim but if it gets to that point, might as well replace it. They don't necessarily cost too much if you get a basic model and DIY.
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I had a toilet a few years ago that had similar problems.
In the end it turned out that my teen age daughter dropped a stick of deodorant into the toilet. Instead of doing the awful thing of reaching into the toilet bowl she decided to just flush the problem away. The stick made the turn around the first part of the S curve but couldn't go any further. It was stuck in the middle of the S curve. It was not visible and a toilet auger would just bypass it. I tied a three prong fish hook to a sturdy string and flushed it. It would catch something but couldn't pull it out.
I finally took the toilet outside and maneuvered a 1/4 inch rope through the S curve and tied the rope to an old fashioned mop head. I attempted to pull the mop head backwards through the S curve and with it the obstruction but this didn't work either.
I still had no idea what the obstruction was. My daughter never admitted to having any knowledge of the missing stick of deodorant.
After a few months of putting up with a toilet that would clog easily I took matters into my own hand. HAMMER! DEODORANT! ANGRY!
After I calmed down I went to Home Depot to get a new toilet. It was only then that I discovered that the size (3.5 gallon) and the color (Harvest Gold) was not available. I settled for a 1.6 gallon white Kohler.
A few years went by with my mis-matched toilet and then my neighbor had a garage sale. In the sale he had his old 3.5 gallon Harvest Gold toilet left over from his updated bathroom with a "free" sign on it. Bingo. I'm back to being matched again with my 70's style bathroom.
P.S. I do not have a avocado refrigerator
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