u/Free-Bag9366

I have a Briggs & Stratton mower. There is gas leaking out of the gas tank from the cap. I originally thought it was a bad seal on the cap, so I'm replaced the cap. Same problem. Looking at the gas it's black like there is carbon in it and there is enough pressure to push it through the seal of the cap. Then I replaced the carb with one from B&S. Didn't fix the problem. Any ideas to what might cause this?

(I have replaced the blade with one that is a bit heavier than the stock one. Not sure if that could cause this.)