While it will still filter, if you've installed it backwards you won't be able to see the build up of dirt & contaminants as they'll be on the inside of the filter element, defeating the point of having a clear fuel filter.
in, from tank
out, to carb
If it's a disposable cheapo like the above & it leaks and/or you've installed it backwards, don't fix it by flipping it around as it'll discharge all the collected gunk into the carb - just;
Originally Posted by 2door
...throw it away and buy a new one...
They're only $1.25 or so: http://www.amazon.com/Replacement-Fi.../dp/B000NEAMBA