I'd also suggest the simple fix of trying a new sparkplug. Several years ago (on Yamaha 660 by coincidence) I had similar symptoms. After stripping and cleaning the carburettor half a dozen times I found I had a dodgy sparkplug. Like yours the original one gave a nice fat spark when removed and grounded, but for some reason can't have been doing this under combustion pressure. A new plug and she ran beautifully.
(I've read of others having similar experiences with plugs breaking down under combustion pressure. I can think of no logical reason why this should happen. If any one has the explanation I would love to hear it)
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