Very nice looking engine bay. From the firewall to the 'L' lever attached to the head looks stock - but the big 'Z' rod (with 90° angles) from the lever to the carbs looks 'very custom'. I'm not sure how to help, other than in a purely mechanical way... Try to make that linkage out of a single rod, that has more gradual bends (no more that 45° if possible). I'm not sure if that will fix your problem, but I'm pretty sure it won't hurt.
So, out of the 'L' bracket on the head, make a ~30° bend a bit below the second corner on your current (90° - 90° bend) bracket. Then make a 45°+45° radius be