Get a syringe with a blunt needle, draw up some ejuice into the syringe. Next, slip the needle between the metal wall of the carto cylinder and the polyfill ( what you are referring to as 'sponge') and
slowly depress the syringe until the juice starts pooling at the top of the polyfill. Wait a minute, and if it isn't absorbed, fill some more; until it won't accept any more.
Then wait a few minutes, 3,5,8, 12 whatever. Then try again. You are probably not waiting, so it's tasting burnt.
The only other thing it
may be is that you are frying the carto with too much wattage or voltage.
What are you using to power the carto? If it's variable voltage/wattage then
that's probably it. Determine what your carto is Ohm wise and use the chart below.