ORIGINAL: NM2K
To me, once you learned to adjust it properly, the only gripe I had about the butterfly carb was the cast-in-place tapered fuel fitting. And that was easy to fix with a file and a lock washer (stopped it from spitting off the fuel line).
Ed Cregger
The butterfly carb worked fine, it just wasn't as flexible as the Mk X. The Butterfly carb had a fixed metering slot. The Mk X had various tapered low speed needles that one could use to fine tune the mid range. Latter models only gave instruction on how to re-profile the low speed needle. These instruction are applicable to any twin needle carb based on the Bodemann model.
By the way Duke replaced all the butterfly carbs on my 1.20 twins with the Mk X every time I sent them in for service.
All the best,
Konrad