Here is the deal, toilet is in the basement. When you flush... the water in bowl rises, and does go down but very slow. Bathtub which is down stream of the toilet flows just fine.

So I removed the toilet and set it on some blocks I the bath tub, I filled up the toilet and simulated a flush - works great so I know the toilet is flowing good.

Then I dumped several 20 liter pales of water down the toilet flange (in the floor) no problem, good flow no back ups.

Then I set the toilet on the flange with the wax seal - I poured the same bucket of 20 liters of water in to the bowl - same SLOW (fill up then drain) issue. (almost seems like a air lock? )

The plumbing had been snaked at least 30 feet towards the city, all good. No other water in the house is slow, just the one toilet. In the basement

I traced the piping. It goes toilet flange - then about 24" then a tee to the bath tub and another tee for the vent line- then another 40" then the sink - then staight to the main drain in the house to the city connection.

Any idease?? I am lost?

Thanks in advance.

The Tee for the vent is fine but the Tee for the tub is wrong, should be a sweep. However, I don't see that this should cause your issue. Have you snaked the vent from the roof? A sink is 1 1/4" drain and the tub 1 1/2" neither of which can match a flushed toilet gpm. Could be a clogged vent and the slower fixtures can handle that but not the toilet.