To clear jams, follow these steps:

Pour a large glass of citric acid down the toilet;
Leave it overnight;
The next day, pour hot water and you will see how the toilet flushes correctly again.
To get rid of yellow marks, here’s how to use citric acid:

Dissolve 150 g of citric acid in one liter of water;
Pour into a spray bottle;
Spray a generous amount onto the affected surface;
Rub with a sponge;
Rinse and dry to remove all traces of moisture.