Why do certain carb caps only work with Terp pearls? Home › Frequently Asked Questions (FAQ) › Information › Why do certain carb caps only work with Terp pearl.... Posted by Steve K on January 14, 2021 This is because the carb cap needs to make a whirlwind or vortex inside the banger. Carb caps for this have holes, slits or other notches in the carb cap to help the air spin.