AAV Causes Weak Flush?

Good morning all, been reading up a bunch on the forums after purchasing a new-to-me home. It's an old house that used to just be an outbuilding so there's a lot of barely legal stuff going on plumbing wise, and I'm trying to figure out what to do for now to stay safe, and what to do in a few weeks when I do a minor bathroom remodel that might allow some better fixes.

The plumbing for my house has no vent to the outside, only an AAV/Studor vent in the basement that comes off the toilet down-pipe. I've attached pictures of the area (I think) below.

I discovered while I was cutting into the drain for a washing machine in the basement that the AAV near the toilet had been taken out so I just have an open pipe into the basement! Obviously a problem. So I went out and grabbed a new AAV and installed it, but when I did, the toilet stopped flushing completely, so I'm betting the AAV isn't allowing enough air in for the toilet to pull its siphon and complete a flush.

Now, like I said, I'm hoping I can run a roof vent while I'm replacing my vanity in a few weeks/months and that will fix the problem better than the AAV, but until then, can you all recommend something so that I can be safe in the meantime? Or is there a good way to install that AAV so that I don't have to run the roof vent at all? It's legal to do so in my jurisdiction but I want my fixtures all to drain right also!

Picture descriptions:
1. The toilet pipe with the AAV stub coming off to the left.
2. You can see the toilet pipe on the left-ish and the house drain running out the right side of the image.
3. New washing machine connection, 24" standpipe, p trap, wye and 45 to a new AAV and drain to the system off right.
4. Sink and bathtub connections running down to the 4" main drain.

Thanks all!

My dear lord i hope you got a great deal on that house. I am just guessing here but it seems like you are getting positive pressure pushing against the toilet flush. The open pipe was relieving that pressure, the AAV is only a one-way valve.

If you take the AAV off and everything in the house works, then if you don't want to take the vent up through the roof the only way to keep the gasses out of the basement would be to run that vent out through the wall and put a 45 on the end of it. If they let somebody run the plumbing the way they did then running that vent out wont hurt anything for now. Let us know what you do. PS You better strap up that pipe that is holding that expansion tank before it breaks something.

CT-18, yeah we got a good deal haha but we knew we were getting a fixer-upper. Any ideas where positive pressure would be coming from in the system?

TomF, I'll support that tank, thanks. I'm OK with running a pipe up to the roof, just gotta work out the best place to install it in the system and pipe it for success. Any ideas on what may be stalling the fixtures with the AAV installed until then? It's cold in VT and I'm just trying to stay safe until it's a little warmer

Tech, I am not sure. One of the general rules is air behind water to help it flow. That is a pretty long drop from the toilet to the main. Its kind of like the milk jug trick, when you turn it upside down to get the water out it chugs. If you put a hole in the jug or angle it to allow air in it drains much faster. Same thing for you. When you flush it sends 3 or 1.5 gallons down that riser that has air in it and that air needs somewhere to go.

I am just going off past experience as each system is its own animal, and I don't do a lot of house plumbing anymore.

Are asking for a route to run it ????? If the closets line up run
it there and just stub it into the attic near your end roof vents
Vent or ridge vent until the weather breaks