If there is any gas flowing out of a Mountain Bike you have serious problems.
In reality you have a Motorcycle, not a Mountain Bike.
It's almost surely a stuck float valve. Try banging on the float bowl with a rubber mallet, but not too hard. You should really take off the bowls and clean them out, then replace the float valve pins and check the floats for any holes or for sticking action. Sometimes the floats will stick open due to rust gunk or old gas gummed up in there. I use Sea Foam fuel additive to help keep the carb clean. You could try running a half bottle of that through a bit, but I'd start by getting that bugger unstuck first.
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