I would start with purple and then progress to yellow. Seems like there were two yellows, a short and a tall, the tall being just a bit stiffer than the short. With the engine off, manually open the center carb fully, and hand operate the back carb to make sure it is not sticking. If not sticking, unscrew diaphragm housing off the carb, push the diaphragm rod up and then put your finger over the vac hole and the rod should stay up, if you have a mity-vac, use it directly on the housing passage and pull a vac and see if it operates and holds vac. If not, you have a torn/leaky diaphragm. You have to test the diaphragm off the carb as there is a kill bleed passage in the carb body that acts like a controlled leak to keep the carbs from coming in too soon.