3/4 inch I will be going by my shop today and I will grab my manual and get back to you. You can e-mail me directly at jeremi_qp@hotmail.com if you need to. Also I asked if you were using an air filter or not, these carbs are set up to open depending on the amount of air flow entering the carbs and if you have no filter it may be opening too much... did you inspect your diaphrams.. be careful not to tear the rubbers as this will undoubtedly effect the operation. -Jeremi-