We've been in the house a little over a year now and the main toilet has never been right. Slow flush, slow draining, poor clearance. In particular, the feeble flow tend to mostly fill the basin, then the level goes down slowly. There's never the "whitewater rapids" effect beloved of toilet duck adverts.
It doesn't appear to be blocked. Chemistry and air pressure have tried, it has become neither better nor worse.
The pipe behind it is about 3' of near-horizontal before exiting the wall and joining the stack. It's not ideal, but I've had worse.
The top of the stack _might_ be blocked? It's old, there's no balloon cage, I've never been up there (3 stories, beyond my ladders) to check.
Haven't inspected the sewers closely, but no trouble elsewhere. The outside toilet works fine, so it's the stack or upstream, not the ground sewers.
Something I've just realised though is that the water level in the pan changes, as soon as you try to flush it. Now isn't this indicative of a double syphonic trap? AIUI, these are less than favoured, for much this reason. So is that likely to be my problem? Is anything in there likely to break? Can I check / fix it? Must I just throw the (ghastly avocado) thing away and fit a new one?
Your advice welcome...