Cistern and Bowl Filling, then Overflow [SOLVED]

[Splurben]

Right, this is strange, but I'm not a plumber.

1. Toilet flushes fine with a ⅓ of a bucket filled with water. It's not clogged.
2. Adjacent bathtub drains fine.
3. Cistern and bowl fill, but at the same time, but not always — this is when the toilet overflows.
4. Flapper is intact, and doesn't leak into bowl…if I fill the cistern from a bucket, no water goes into toilet bowl.
5. Only occasionally, when cistern is filling, both the cistern AND the bowl fill simultaneously.
6. When the cistern and bowl fill simultaneously, the toilet overflows.

Not a plumber, but there's something here that I'm not understanding.

Here's a video to show what's happening in shocking detail:

 

[WorthFlorida]

The fluidmaster fill valve is bad. By the looks of it remove it and replace it with a new one.

If not you remove the top part of the valve and replace the seal or diaphragm. There are plenty of u tubes videos on how to do it.
https://www.fluidmaster.com/products/toilet/fill-valves/400a-universal-fill-valve/

 

[Sponsor]

 

[Terry]

I'm not convinced that the bowl isn't clogged somewhat. You can pour the bucket in, but try it with wadded up toilet paper and see what happens.
Even pouring the water directly into the bowl, it's looks to be a pretty poor siphon.

Does the fill valve ever shut off? The video seemed to end before we could see that.

 

[Splurben]

I'm not convinced that the bowl isn't clogged somewhat. You can pour the bucket in, but try it with wadded up toilet paper and see what happens.
Even pouring the water directly into the bowl, it's looks to be a pretty poor siphon.

Does the fill valve ever shut off? The video seemed to end before we could see that.

When it overflows, the fill valve doesn't shut off even when the cistern pushes the float all the way up. If I were to leave the water running until the fill valve shut off, the bowl would be overflowing. If you'll pardon the pun, it's a crappy old toilet (although the internal flush mechanism was replaced last year).

 

[Reach4]

If you'll pardon the pun, it's a crappy old toilet

Rather than invest more time on the old toilet, get a new modern toilet. Despite using less water, they flush better. They seldom need a plunger.

 

[Splurben]

The fluidmaster fill valve is bad. By the looks of it remove it and replace it with a new one.

If not you remove the top part of the valve and replace the seal or diaphragm. There are plenty of u tubes videos on how to do it.
https://www.fluidmaster.com/products/toilet/fill-valves/400a-universal-fill-valve/

This fixed it! Thank you WorthFlorida. I ended up getting

— spent about $12 with shipping — and everything's working perfectly now. Thank You WorthFlorida