When depressing the trottle, why does gas pour from bottom of carburetor, causing it to slow and backfire?

You may have more than one problem here. The over fuelling when you press the accelerator is likely from an accelerator pump that supplies an additional amount of fuel to accelerate the engine. Some carburettors have an accelerator pump jet that can be directed. If the direction of the jet is wrong the fuel will hit the throttle butterfly and be diverted, The jet should start to operate as the butterfly is about one third open and the jet should squirt past the butterfly and into the throat of the carburettor. The backfire may be another problem, either engine timing or carburettor mixture but could also be too much fuel as you describe. The carburettor float level needs to be exact. Sounds like someone may have had the carburettor apart with no knowledge of how it works? It could also be caused by low compression and poorly seated valves. Do not be confused by 'backfiring' and 'popping back'. Popping back is caused by a lean mixture and generally follows a rapid closing of the throttle, whereas a backfire is caused by an over rich mixture which allows unburnt fuel to enter the exhaust system, which is then ignited by the hot gas from the next combustion stroke. If I were you I would take the carburettor off and do a good job on it using a proper overhaul manual and a carburettor kit. Set it up meticulously or give it to a Pro.

Rgs. Bob

Sounds like over fueling... check gaskets on carb.