On 3/24/17 3:02 PM, Bob wrote:
I just bought an old farmhouse (really good deal) an am going through
all the repairs that need to be done and now have a puzzle.

When a large amount of water is flushed down the pipes (i.e. bathtub
emptying) the basement fills with sewer gas. Found a clean-out cover
rusted out (and now fixed) but still having the gas odor.

Some cut.

There are these if venting is a problem. Air admittance valve.
https://www.amazon.com/s/ref=nb_sb_noss_2/159-4519604-7296749?url=search-alias%3Daps&field-keywords=drain+vent