If your van really has a carbeurator it sounds like either an accellerator pump, or the metering wells are leaking the fuel in the float bowl. A carb rebuild kit should take care of your problem. If the metering wells are leaking there used to be a kit for the four barrels to fix that, as there should be one for two barrels.
You should have electronic fuel injection though
If you do the injectors may need cleaned, or you have a vacume leak at the throttle body.