odd flushing toilet

Is the water level in the bowl and the tank the same for both flushes?

How about a third flush? What's it like?

I'm thinking there's a leak, but not enough to get the valve to open. So there's less water in the tank when it's been sitting for a while. When it's freshly filled the tank is full and you get more water.

Pressure in the building could be pushing some of the water out of the bowl over time too.

What if you hold the lever so it empties the tank on the first flush? Not quite sure what that tells you but it is another data point.

The flapper or other flush mechanism in low flush volume toilets is designed to drop back in to the seat long before the tank is empty usually by when the air leaves the bulb in the bottom but this is definitely something to think about. It is going to make the water level dropping an inch or 2 because the flush valve isn't holding a lot more noticeable.

the little pilot jet hole at the bottom of the bowl where it transitions to the throat of the trap, down there in the dirty,

mine got clogged heading back up to the rim, when yellow mellows it can crust that small pilot channel,

I ran a snake up there, actually some 3/8 vinyl tube, and broke thru,

been flushing like a champ since,

might be worth checking.

could be something either in the passages in the bowl or in the drain that is floating around

If the toilet discharge pathway gets reduced in size by urine salts, you won't get the siphon action you need for a proper flush. Put a glove on and reach down in there and feel the "G" spot and if there's a buildup of deposits, chip it away with a bent spoon or other tool.

I've used Ed Fred Toilet Bowl Cleaner with good results. Remove all the water in the bowl, pour the entire bottle of Ed Fred in and let it sit overnight. I only have the one toilet in my house, so I had to go in the garden in the middle of the night when I did it. Flush in the morning and scrub that channel with a brush. A second application may be necessary.

assuming the flooring goes underneath it

Fill a bucket with water and dump it down the bowl. If it flushes it's probably not the bowl but the tank. Be sure to check that small ping-pong-sized hole in the bowl for an obstruction.

Does it have a cylindrical flush valve or a flapper? I'm guessing it's a flapper type with a float on the chain. Either the chain or the float, or both need adjustment. And try adjusting the ball cock to create a high water level.

If all else fails change the toilet to a Kohler toilet with a cylinder-type flush valve and be done with it. The low-flush toilets can be a bit of a pain with this type of thing.

Will never go wrong with a Toto Drake series tank/bowl

Model numbers don't come to mind. The big box hardware stores have them in large single boxes that come with pretty much everything you need.

Mansfield had one. Still does.

Yes! Very nice lol. But let us be sure not to mistake the use of one for the other. Or has that already happened?

Would anyone know if that Aqua Piston flush valve can be used to replace the common flapper valve?

If yes, is this true for all brands?

This type doesn't siphon the trap. I saw them in Japan when I was there- not very impressive. Toto does make one with the same rim jet setup where the trap does siphon- I'd use that one.

it moves the 💩 with zero clogs, what more we could ask. 😂
PS: that is also the older version with rim type flush through lots of holes. Newer one jets all water from rights side in channel and scrub the bowl with stronger swirl.

funny they all seem to make the gulping sound though.

I have 2 Kohlers 1.6 gph about 15 years old. One is round the other elongated. They both flush good but the round one is better.

I changed one in my nephews rental property. I put in an American Standard cheap HD toilet. Forgot the model 1.25 gph and it flushed great.

Cast iron?

Curious if replacing it with 3" PVC would fix the issue.

Your pipe is less than 1/4" per foot but more than 1/8 so it still passes modern code.