Snaping open the front two barrels would normally depend on a good accelerator pump shot of fuel, to avoid bog... Like Cliff said.
Snaping open all 4 barrels would require the vacuum diaphram to let the secondary flap open in a smooth, timed, movement. Shove the plunger in and hold it shut with your finger over the hole, then let go as you count off seconds. 2 seconds or more should work fine.
Also very important is the amout of spring tension on the secondary flap. Try 3/4 of a turn tension setting on the flap spring.