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 bend upward to the 90° to the carb linkage. You can use a tubing bender, but might need to heat the rod a bit to allow for good bends.
I attached a picture of my '48 D24 (the return spring and hand throttle are currently disconnected), but it is a single carb. Maybe try something similar, where you go more stock out of the bracket on the head and then try to make a lever on the two carbs that comes down to the rod coming off of the head assembly. In other words, try to make the rod coming off of the block lever 'flat' like that going to the single carb and then make something that connects the two carbs together.