Studor valves only let gas (air) IN, not out. Your vents need to go OUT (well, mostly) to let the sewer gas escape.
The vent system, if it is working properly, allows the sinks, toilets, etc. to drain properly. The traps in those devices keep sewer gas from sneaking in the house. Assuming thatyour vent system is installed properly, the sewer gas is coming in from an open window or other air vent.
Check each sink, toilet, tub, etc. and see if you smell anything coming out of it. You probably won't. But, if you do, a faulty or blocked vent may have allowed the water thatis in the trap to siphon out.
I'm not a pro, but it seems thatthe vent is supposed to be high enough so that the top is 2' above the roof peak. You might want to double check that value, but just like smokestacks, raising the outlet gets it into the airstream and carried away better than getting caught in the air current eddies often present on a roof.