FWIW I had trouble finding an original Sum in decent condition and bought a new repro only reluctantly. I've since bought an original but it stays in a drawewr - the bike runs much better with the new version. I found that the jetting wasn't quite right on the new carb and I had toplay around with them but once I'd bought a new (accurate?) set from Mark Huggett the bike was transformed .
That's what I intend on doing when they are back in stock. The bike was a complete basket case from a shed in France and came with an old bing that was corroded. I've now got it running on an IZH 350 carb which I had as a spare for my DKW NZ 350. Its running really rich though.