New 2nd floor toilet won't flush consistently

[Jrea]

Hello Everyone. We purchased my inlaws century home and I have been trying to correct some of the plumbing issues but this one has me stumped. 2nd floor toilet in half bath will not flush consistently. Original toilet was only used for minor transactions because it would not even flush toilet paper. I discovered that the main jet was nearly plugged solid with calcium deposits. I cleaned it out thoroughly and was able to achieve a relatively full consistent flush. I tempered fate and used it for what it was made for, it flushed fine at first but then it overflowed a couple of days later after attempting the same load. I ended up purchasing a brand new Kohler toilet (was in the plan to replace it anyway) and it has the same problem. It might flush seemingly normal once but then the water just fills the bowl and slowly trickles out. I disconnected the sink drain at the wall to see if there was a venting issue but that changed nothing. I also ran a small snake about 15 feet down the drain when the toilet was removed and felt no obstructions. The sink drains just fine. Any ideas? Thank you

 

[GReynolds929]

Sounds like there's a partial clog in the line.

 

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[Reach4]

Try this: Get the biggest container that you can manage, and pour the water into the bowl from as high as practical, as fast as you dare-- until the water in the bowl starts to rise. See if the deluge clears the way.

Kitchen waste can, 33 gallon garbage can, or whatever.

A drain cleaning specialist can use a bigger cleaner than your little snake that could poke right thru a soft clog.