ORIGINAL: earlwb
The fuel pump provides for more pressure than muffler pressure does.
Generally the pump carbs have a larger bore inside as the pump provides the fuel instead of the engine sucking the fuel. Plus they have a slightly different mid range trim in the carb to account for the extra fuel pressure too.
The fuel tank is rarely inline with the carb as it is usually a little lower. So then the plane goes inverted the fuel flow increases causing the slight rich running condition. The fuel pump tends to aggravate the situation. So yes the slide valve would work for that. But the carbs are setup for pumps, so you probably wouldn't find a non-pump Perry carb with that feature on it.
The reason I asked is a friend of mine has an old K&B, I think it's a.60 or.61?with this carband pump but the pump is shot so he set it up with exhaust pressure to the tank, it runs fine when upright and stationary but as soon as he points the aircraft verticalit quits and whenit is held inverted it goes blubbering rich.
I think he's looking for the K&Bcarb.
Happy flyin' Oscar