I just replaced my old toilet with a new one but when i came to test the installation by flushing, the water would only go down slowly. I had replaced the seal. I took it all apart and ran water down the open sewer pipe and it ran freely. I put everything back together with another new seal and flushed again the same thing happened. I wonder if you can explain my problem.
Your test of pouring water down the drain shows that the sewer drain itself is open. The most likely cause of the problem is a blocked vent. Inside the toilet is a trap to prevent sewer gasses from coming back up into the home, and that same trap prevents air from being flushed along with the water. When the toilet flushes, air must be introduced into the sewer system to allow the water to flow; otherwise is it like holding your finger over the end of a straw and lifting it out of the soda … the soda stays in the straw until you allow air to come in the top of the straw.
The vent system should terminate through the roof near where the toilet is located. The vent pipe may have debris, or even animal nests in it (squirrels and birds are often the culprits). Sometimes, a vent cap has been crushed to block the flow of air into the vent. You can safely use a plumber’s snake from the roof to dislodge or clean any debris from the vent line.