I'm flying C172s, and I have a question. During flight if a fuel vent (located on the left side) becomes blocked due to a bird strike or icing what should I do? Are there any auxiliary fuel vents?
Cessna 172 fuel tanks are vented on the left side through a port, and the caps are also vented. The tanks are interlinked as long as the fuel selector is set to both, so a vent blockage on one side should not cause a vacuum. This makes a tank vent issue very unlikely, but still possible.
If all three vents were to become blocked there's nothing you can do about it, as the fuel caps are on the top of the wing you may not even realize there's a block at all until your engine quits due to fuel starvation. If you do suspect your vents are blocked you should declare an emergency and start looking for a place to land.
Keeping in mind vent blockage is very unlikely, if you have a reduction in engine power you should follow procedure, i.e. fuel pump on (not applicable to the C172 but you should memorize it anyway), carb heat on, fuel tank change (again not generally applicable to the C172 as most of the time you'll be set to both but still memorize it), check mags are set to both, mixture rich, etc.