sump type and sniffle valve
My understanding is that a sniffle value is only useful on horizontal induction. A vertical sump will allow the fuel to drain out the intake. The fuel will remain "trapped" in the horizontal sump, particularly on taildraggers.
The sniffle valve simply opens when there is higher pressure on the inside of the sump than outside it. This is usually only the case if there is some buildup of fluid (normally fuel) inside the sump. When the engine is running the sniffle valve should close due to the lower pressure in the intake system.
If there is fuel trapped in the bottom of the intake, it can make starting harder as the engine is flooded. There have also been cases of the fuel igniting (backfire), and damaging the composite sumps that Superior used to sell - they called them "Ryton sumps".
If you are worried about flooding the engine or backfires, then you probably want to have a sniffle valve. I've got one, not flying yet, so can't really comment on if it is useful for my particular installation.
If your valve is sticking, probably worth it to take it off and have a look to clean and test it. I have not cut one open (yet) to see how they work (they are kind of expensive) but expect that they are quite simple.
