[blykins]

Well, its not always evident....could be an inner or damper spring broken, doesn't have to be a broken piece laying out in the open.

Not saying that's what's wrong, but it's something simple to rule out. The stumble and loss of power could be valve float from a spring giving up.

If you wanted to try something quick on the carb, you could jet up quite a bit and see if that affects it. On the dyno with these carbs, we normally just have to do a slight HSAB adjustment if anything at all.