Weber Idle Air Balancing
Posted: Tue Feb 22, 2005 8:48 pm
Decided yesterday to go back and replace the butterfly I had to file on the outer profile inorder to get it to fit at all that I installed about a year ago. Bought two more 78 degree butterfly discs and proceeded to install them yesterday an immediately ran into a problem. They differed by .002" on the major axis so when one was tightly closed the other was hung open. I got had again by more official Weber spare parts being junk. My only recourse was to do what I advised just the other day should not be done and that is twist the throttle shaft. But this is not why I'm posting.
Went on and drilled the tiny air bleed holes in the butterflys to do the idling airflow balancing which must be done over and over again until you guess it correctly. What a royal pain! Realized what is needed is a jet that screws into the butterfly to make this tuning process even more user-friendly. Which led me to consider modifying my carbies so they have the air bleed bypass circuit with an adjustable screw just like the 151/152 version has. For the effort to make a selection of different jet sizes I can just build the bleeders. The air must be routed around the butterfly so the airflow can be measured with precision and the airflow made the same in all the throats. What this does for you is now you reduce or increase the airflow on all the throats together so as to exactly position the edges of the butterflies just downstream of the first progressive holes so there is no lean hole. Drilling the holes does not do that. You can only make them larger which closes the butterflies even more moving them way from the first progressive hole. One mistake and you start over again.
Got it fairly close with drilling the holes but it's a tremendous amount of labor to do it this way. It purrs now. I know with the adjustable air bleeders I could do this in a matter of minutes rather than taking all day. They're on my to-do list.
Went on and drilled the tiny air bleed holes in the butterflys to do the idling airflow balancing which must be done over and over again until you guess it correctly. What a royal pain! Realized what is needed is a jet that screws into the butterfly to make this tuning process even more user-friendly. Which led me to consider modifying my carbies so they have the air bleed bypass circuit with an adjustable screw just like the 151/152 version has. For the effort to make a selection of different jet sizes I can just build the bleeders. The air must be routed around the butterfly so the airflow can be measured with precision and the airflow made the same in all the throats. What this does for you is now you reduce or increase the airflow on all the throats together so as to exactly position the edges of the butterflies just downstream of the first progressive holes so there is no lean hole. Drilling the holes does not do that. You can only make them larger which closes the butterflies even more moving them way from the first progressive hole. One mistake and you start over again.
Got it fairly close with drilling the holes but it's a tremendous amount of labor to do it this way. It purrs now. I know with the adjustable air bleeders I could do this in a matter of minutes rather than taking all day. They're on my to-do list.