The vent sounds ok. The 90 degree bends does not sound unusual and should not cause a problem as long as everything is sloped to drain. Are there any other water using fixtures in the bathroom? Is there a sink and does it have a proper drain? Is there a tub and does it get used? Have you checked to see if everything has proper traps? Are there any clean outs in the bathroom? What about floor drains? Could the odor be coming from an outside air intake? Are there any air vents or chimneys in the room? Have you tried to sniff out exactly where the odor is coming from? Are there cracks or areas that are difficult to clean that could be the source of the odor?
As far as noticing the odor more after a rain storm, this may be from an increase in humidity which can affect odor in a room.