Cause of poor toilet flushing

[brb]

Hey everyone! I am having issues flushing my toilet in the basement of my house as in you have to hold the lever down until the whole tank empties into the bowl. I am adding a 1st floor bathroom and discovered that the basement bathroom empties into the CI 4in main drain with a 45 wye right after it passes through the back flow preventer. See pic 1. I was installing a new back flow device and decided to just try the toilet to see what happens. Turns out the water is running back up the pipe to the dirt as seen in pic 1 with the gap between the pvc and the other fitting. . I had it all apart today and am wondering if the fitting needs to be changed to a 4in with a long sweeping fitting like in the picture 2. The pipe going to the street is clear from what I can see. The basement bath is vented with a 2in pipe but I can't see exactly how it is vented in the walls etc.

 

[Dgeist]

Have you checked the flow on the CI drain downstream with anything other than the toilet? You may be encountering a blockage that needs to be cleared and the toilet's flow is just a hapless victim. That long sweep enters the wye at the same angle as the other one. You might get a tiny bit more velocity on the slug, but depending on the problem, that might not make anything better.