I have 2 family house where 1st floor drainage system working fantastic. Now, 2nd floor bathtub and kitchen sink is connected to my basement bathtub drain. My basement bathtub backs up sometimes and after a while the water goes away. Sometime I see some bubbles in basement bathtub when someone take shower in second floor. I ran 50ft drain snake through my basement bathtub and nothing is stuck. I’m guessing it’s some kind of venting problem. Anyone know how to fix this issue without calling a plumber and paying lot of money?
This doesn't sound like a venting problem.
If your basement bathtub is backing up, I assume it is the lowest point in your plumbing drainage piping. This typically means that the sewer service line to the city is clogged or restricted.
What snake did you use? How far from your property to the city sewer main? 50' doesn't sound like enough. How old is the house / sewer line, do you trees on your property? You might also want to rent the scope to see the problem.
Rent the 100' sewer snake from homedepot and run that from your house out to the sewer main.