Try getting the tank pumped out or put some chemical treatment down it
perhaps it is gotten inactive due to lack of use
Tater wrote:
I just bought an old farmhouse an am going thru all the repairs that
need to be done and have a puzzle.

when a large load of waster is flushed down the pipes (i.e. bathtub
emptying) the basement fills with septic gas. found a cleanout cover
rusted out (and now fixed) but still having the gas.

I'd say that when the water enters the tank, the gas gets pushed back,
after the fix it has dropped noticeably, but it appears it has now been
directed to the tank vent.

still some gas in basement, and a fairly good stinker from the septic
vent, have i been living in the city soo long to forget that this is
normal?