Boxerworks.com

Really interesting that the slides are not moving on either carb. Now I am experienced with SUs found on English cars. They have a port on the facing of the intake horn of the carb that if blocked will not allow air to get into the chamber allowing the slides to move up. Is there a passage that allows air to get into the chamber above the diaphragms??
Have you run a wire into all the holes in the body of the carbs. The new and improved fuel we have now a days, when allowed to dry out, leaves a hard varnish that blocks passages.

There is a passage through the slide to above the diaphragms. The air rushing through the carb throat creates a vacuum above the diaphragms which thus raises the slides. Could be the slides are stuck in their bore or the passage way is blocked or the diaphragms are holed.

Kurt in S.A.

I think your fly is open....

They move up and down fine with my finger. I ordered a leak down tester today. When I ran the bike with pipes from the airbox removed from the carbs, I saw some backfiring in the right carb. That makes me think that the right intake valve is either still partially open at time of ignition or that the valve or valve seat has issues.